I ran the Half Life 2 Counter Strike Source video stress test on my machine:
Mac Pro '08
4 GB RAM
GeForce 8800 GT
Ran the stress test on both Win XP Pro 32 bits SP 2 with all updates, and latest nVidia drivers, versus Vista Ultimate 64 bits SP 1 with all updates and latest nVidia drivers...
Both tests done on the same fysical HD, 1st 160 GB NTFS partition for XP, 2nd 160 GB NTFS partition for Vista.
Vista is 5 fps faster, on this DirectX 9 game (265 fps vs. 270 fps).
Not a lot, but it is a difference which I could regenerate every time.
But... I do use XP for playing older games like DTM racer 2, Richard Burns Rally, etc. because those games have issues on Vista. (bad force feedback support, or random crashes etc.)
If you have a DirectX 10 compatible grfx card, I wouldn't dream of running Crysis on XP!! Must-see-grfx-on-DirectX10-on-Vista! Runs gr8 on Vista Ultimate 64 bits!