For a tall man, Tim Armstrong has been on an awful lot of online companies’ short lists.

For a big Web exec job, that is. Indeed, whenever one opens up in the Internet space, the 6-foot 3-inch Google ad sales exec always pops up on it as a possible candidate to lead a variety of digital companies and start-ups.

Finally today–after longtime speculation that Armstrong had long wanted and would eventually leave his post at Google (GOOG) in order to try his hand at being top dog–he took over as chairman and CEO of the once-mighty, but now-not-so-much, AOL.

Armstrong, 38, will start at AOL on April 7.

“For me, it is a great opportunity to go to what I consider a top-five Internet brand,” said Armstrong, in an interview with BoomTown this afternoon, with a whole lot of the diplomacy and nice-guyness he is well known for at Google and in the online advertising industry. “I am looking forward to taking what I have learned at Google and seeing what I can bring to really help AOL.”

Noting that for all its decline–pointed out by me–AOL was still one of the few “global Internet brands,” Armstrong said he thought there was still a lot of juice in the consumer appeal of AOL.

So much so, he added, that AOL owner Time Warner (TWX) has given him a lot of options for its future, from keeping it inside the larger media conglomerate (unlikely) to partnering with another company (less unlikely) to spinning it out (likely!).

“One of the things we discussed was making sure we were able to have the best outcome for AOL,” said Armstrong. “That could take the form of a lot of different paths.”

(Translation: As soon as the economy brightens, I am going to become a public company CEO, just like my soon-to-be-ex-boss Eric Schmidt!)

The move to put Armstrong in at AOL was sudden and swift, and also more than a little cutthroat on the part of his new employer, which bounced current Chairman and CEO Randy Falco and President and COO Ron Grant without a lot of warning to them or any top exec at AOL.

While there has been much talk about when Time Warner would become weary of the pair’s management of AOL–which has been rocky (most especially their overpaying for the Bebo social-networking site, which others at Time Warner never got over)–their defenestration and Armstrong’s installation happened rather quickly.

And, indeed, Armstrong confirmed that the talks to take over at AOL had only started a few weeks ago, increasing in “intensity over the last week.”

So intense, for example, that Grant only found out he was being replaced this afternoon after a personal visit from Time Warner CEO Jeff Bewkes, who made a rare appearance at AOL’s downtown Manhattan HQ to deliver the bad news.

(One AOLer’s funny, but entirely imaginary, vision: Bewkes signed up Armstrong, whose Google office is right nearby AOL in New York, and then hightailed it over to AOL to drop the hammer before the ink was dry on the contract.)

Via a coup or not, nabbing Armstrong is indeed a coup–at least from a shiny resume point of view–for Bewkes, who has been struggling with what to do with AOL for a while.

While he often affably jokes about its many problems–from declining ad sales to management turmoil to, it must be said, increasing irrelevance–Bewkes has been trying to sell off AOL or turn the asset into something more valuable for far too long.

Bewkes knows Armstrong well, as Google is a major partner of AOL in search advertising, and Google also owns five percent of AOL, in a deal in which Armstrong was involved (and whose value the search giant recently marked down).

Armstrong said he was also close to Time Warner General Counsel Paul Cappuccio.

“It was a natural fit with AOL, since I know the company so well,” Armstrong said, adding he would spend his first weeks getting to know AOL’s employees and its products better, before making more concrete strategic decisions or changing any course setting of Falco’s.

Under Falco’s plan, AOL was focusing on a three-pronged strategy: social networking and communications (People Networks), content (MediaGlow) and advertising (Platform-A).

“In general, I do want to spend time with the staff,” he said. “Some of the stuff I have seen so far has actually paid off…and a lot of the new products show a lot of passion.”

Whether he can turn passion into blockbuster products is another story, and some are worried that Armstrong’s experience is too heavily weighted in ad sales rather than in development of killer services, which is what AOL might need to recover.

But Armstrong said he had a lot of other operational duties at that search giant, noting that “Google is a very complex business.”

And both current and former AOLers hope his ad experience will allow AOL to return to its strong premium advertising roots that were less focused on of late. In fact, Falco recently hired former Yahoo (YHOO) sales head Greg Coleman to do just that.

And Armstrong has a lot of support from unusual sectors too. Wrote former AOL head Jon Miller, who was, ironically, forced out by Bewkes in favor of Falco and Grant, to me in an unsolicited email: “Count me amongst the Armstrong fan club.”

And, many staff at AOL I spoke to today–whose morale has been buffeted by layoffs and ongoing bad news–seem genuinely thrilled to score such a prominent exec.

“I am so thrilled. We couldn’t change the DNA with Rondy on top,” said one exec, referring to the derisive nickname that Falco and Grant had within AOL, which combined their two first names. “I feel really positive.”

So does Armstrong. “I am really looking forward to running AOL,” he said.

And personally, as a longtime and clearly obsessive watcher of AOL, I am looking forward to seeing him try.

But it took a lot of guts for Yahoo President Sue Decker (pictured here) to resign immediately yesterday and without the usual polite waiting period that happens in most major corporate management shifts.

I am guessing Decker clearly knew that investor ire over her leadership and that of her boss, Yahoo CEO Jerry Yang, had made the duo radioactive. Yang had already said goodbye in November and now it was her turn.

It was an important move for Yahoo (YHOO), since questions of Decker’s continued presence would have definitely dragged down any excitement to be had about the announcement of new Yahoo CEO Carol Bartz.

So, Decker–who has always been a clear-thinking analytical type more than a gut executive–made both the logical and emotional decision to leave the company she has worked at for nine years.

And, more importantly, to leave Bartz with the cleanest slate she possibly could.

Whatever you think of Decker’s leadership, it was a class move on her part and one that BoomTown admires, especially since it was probably very hard for such a rigorous person to walk away without leaving behind a success.

It would be nice, then, if the man who was also just as–if not much more–responsible for the disaster that has been Yahoo for too long would take a clue from Decker and Yang and head out too.

That would be Yahoo Chairman Roy Bostock (pictured here), the leader who has largely escaped blame that has been massively heaped on both Yang and Decker.

But it was Bostock who was part of the supposed revival of Yahoo, who was a key player in the disastrous negotiations with Microsoft (MSFT) over its takeover attempt and who was has been pulling many of the key strings the entire time.

Thus, he needs to go too, because at this point, Yahoo has to declare that accountability matters and that it starts at the tippy-top.

Major investors certainly would not mind. “Everything he has promised has gone the other way,” said one about his encounters with Bostock. “I just don’t believe him anymore.”

Said a former Yahoo exec, echoing the sentiment of many I have talked to over the past weeks: “Jerry and Sue should be the ones to take responsibility for Yahoo’s decline, but Roy was right next to them all the way.”

So, while many could aim at Yang and Decker (or perhaps even former CEO Terry Semel) or the distraction of the Microsoft bid earlier in 2008 or now, the weakness in the economy for Yahoo’s woes, since he has been chairman, Bostock is just as responsible as anyone at at the top of Yahoo.

Yahoo shares have been cut in half since he took over as chairman a year ago, back to where they were when he joined the board in 2003.

You could also point to Yahoo’s morale crisis or the exodus of key talent or the declines in its key businesses or the company’s general lack of momentum as very solid reasons for kicking Bostock to the curb for taking so long to do anything about it.

BoomTown will not even bring up the $31 per share offer from Microsoft that Yahoo spurned, although Bostock never seems to tire of highhandedly laying all the blame for the mess at Microsoft’s feet alone.

While the departure of Bostock would probably be enough, I would say a lot of the longtime members of the board should also probably contemplate moving on, to make way for some fresh eyes on the board and to free Bartz to do what she wants without their meddling.

Given that several directors also wanted the CEO job, according to sources, the board will likely continue to be more bothersome than helpful.

While not the boldest pick by this largely ineffectual and slow-moving board, it has finally made a definitive move by picking Bartz and should now get well out of the way.

Because, unlike the Yahoo directors, Bartz cannot really be judged yet, and one must give her the benefit of the doubt and let her try to fix Yahoo in her own way.

She certainly has the experience as a seasoned tech veteran, from 14 years at Autodesk (ADSK), and she also has the goodwill of Silicon Valley.

Left to hire her own team and take advantage of the massive amount of talent that still miraculously exists at Yahoo, Bartz has every chance of reviving a company ripe with great assets.

That is, if Bostock–who led the search to hire her–would give her a truly great gift of the cleanest slate possible for Yahoo, by bidding her farewell and godspeed as he heads out the door.

From our Monday post called “Say Hello to the Yahoo Board Members”:

Recently, [former CEO Terry] Semel revived his Los Angeles-based new media investment firm, Windsor Media, and rumors abound to his intentions–including possibly making a play for a Hollywood studio. Big question: Will Semel continue as chairman of Yahoo in 2008?…

“What to make of Roy Bostock, who has been on Yahoo’s board since 2003? I’ll tell you what: If Semel were to step down as chairman, the chatter is that the former top-level advertising exec (chairman and chief executive officer of D’Arcy Masius Benton & Bowles) is best suited to the job, given the importance of Yahoo’s ad business.”

From a press release today by Yahoo:

Yahoo Inc. (Nasdaq:YHOO), a leading global Internet company, today announced that Terry Semel, the company’s non-executive chairman, will step down from the Board of Directors, effective Jan. 31, 2008…

“Yahoo also announced today that Roy Bostock, who has been a member of the company’s Board since May 2003, was unanimously elected by the Board to serve as Yahoo’s non-executive chairman.”

Our next prediction: Yahoo has exclamation point surgically removed until happiness returns once again to Sunnyvale.