They were going to have a rough spot right now whether they bought ATI or not. If anything, they waited a little too long as their share price had passed its peak. But then the evidence is the discussions started December 2005, and it took about 6 months to get to closure.
The interesting question is if they were determined on this course, which I think it is clear they were (come on now, everyone who thinks Hector woke up one morning, looked in the mirror while shaving, and thought to himself, 'Hmm, today seems like a good day to spend $5.4B on something I don't really need. . . wonder where I can find something like that? Think I'll ask Dirk if he knows of somewhere', please raise your hand), then were there other options that could have worked out? Maybe a VIA, a SiS, or even both?
I don't think buying ATI was a mistake per se. However, IMHO it was a case of "too soon, too much". Not to sound like a parrot, but I think they could have saved themselves a lot of money had they waited a couple of quarters, and I am not just saying it with benefit of hindsight.