FiringSquad looks at the performance of Vista and XP from a gamer's perspective. Apparently, things have improved quite a bit:

Both AMD and NVIDIA’s Windows Vista drivers have come a long way in the past seven months. NVIDIA in particular has made tremendous strides with their latest Vista driver, SLI support is fully functional for all GeForce card owners and it scales well in most cases. Unfortunately, CrossFire compatibility is still an issue for AMD. New games like BioShock and World in Conflict don't support CrossFire at this time, and Lost Planet and Quake Wars have graphical glitches.
If you were debating between the 32-bit and 64-bit flavors of Windows Vista, fortunately it looks like performance is similar with either version. Both AMD and NVIDIA’s drivers for both versions of Vista perform practically identical to one another. And if you were concerned about game compatibility with 64-bit Vista, one of the guidelines Microsoft has required for Games For Windows certification is that games must be compatible with Windows Vista x64. This means if the game has a Games For Windows logo on the box, it’s been tested to run with 64-bit Windows Vista. Upcoming games like Alan Wake, Crysis, Fallout 3, Gears of War PC, and Hellgate: London are all Games For Windows compliant.

Considering all this, we’d recommend our readers opt for the 64-bit version of Vista if you’ve got a 64-bit CPU. It runs just as fast in games with the added advantage that it’s more secure and can address considerably more memory (4GB max in 32-bit Vista versus 128GB in 64-bit Vista Ultimate).

OK, so that's fascinating. Incidentally, I'm planning an article, tentatively called Windows Vista: One Year Later, for early November. One of the primary goals of the article is to discover whether the x64 value proposition has changed. That is, does it make sense to even consider running a 64-bit version of Vista yet? It certainly didn't when the OS first shipped. More on this soon.