TomsHardware has written up an article about the performance of Microsoft's newly-released Windows Vista. The article uses a few game benchmarks (including UT2004) as well as a few audio, video, and application benchmarks:

Our hopes that Vista might be able to speed up applications are gone. First tests with 64-bit editions result in numbers similar to our 32-bit results, and we believe it's safe to say that users looking for more raw performance will be disappointed with Vista. Vista is the better Windows, because it behaves better, because it looks better and because it feels better. But it cannot perform better than Windows XP.