Depending on the game, you'll get similar or better results under windows XP. Cases when it's true:
- the game was originally made for DX9, and 'ported' to DX10: much faster, not much lower visual quality
- the game was made for consoles first: today's consoles don't go past DX9 capabilities
- the game was first programmed to run in OpenGL and retains that rendering (main or optional) in its final version.
In all those cases, there is also the fact that running a game under WinXP allows you to remove much of the OS bloat - something that can't be done in Vista.
For those Vista fanatics that would claim otherwise, a stripped down WinXP uses very little RAM and CPU and I/O resources - a stripped down Vista goes from slow as an arthritic turtle to slow as a thirsty snail, and from bloated as a water balloon to bloated as a 80% filled water balloon.
And yes, I did tinker heavily with both.