Vista 64 here: I'm sure there's got to be a few frames per second here and there. But in actual usage, I do not notice any difference with the 32 bit games I have installed on my system. Warcraft 3, World of Warcraft, Neverwinter Nights 2/Mask of the Betrayer, Oblivion, and Bioshock. Having said that, I do have a solid amount of horsepower and up to date components inside the case on my desk.
More to the point I think you're after tho: I do not recommend installing Vista in any flavor on an older system. Drivers, while *hugely* better than they were even a few months ago, can still be somewhat finicky at times. Often for difficult to determine reasons, as well. And I've noticed that many makers aren't even bothering to create drivers for older components. They'd rather force you to buy new. So I'd recommend that if you have XP now, you should stick with it. The benefits aren't all that great since XP also uses multi-core processors. Reportedly not *quite* as efficiently as Vista, tho in practice it's certainly close enough. You're basically gaining a prettier interface and DX-10 for the money and the trouble of upgrading.
Of course, it's a different story for a new build. Yes, there are some issues, mostly related to vendor's drivers and incompatabilities which haven't been worked out quite yet. A little bit moreso if you're discussing V64. But if you look at the two at their respective releases and at similar stages in development, Vista *is* better than XP was.