Well the answer has 2 points ...dx 9 games are supposed to perform slightly better on Vista ..But, other people say that the DRM scans your hardware 30x a second and decreases performance ...so onto the main benefit:
Vista ships with DX 10 ..So yes, games like MS flight simulator (dx 10) will perform much better. But here is the catch... All the current dx 10 games have been delayed ..even the dx 10 "patch" promised for MS flight simulator.
So vista will provide awesome gaming ...but that benefit is receeding past the horizon.
Perhaps in the summertime
edit: and in case you are still unaware --DX 10 requires both Vista and a DX 10 compliant video card (like the nvidia 8800 series)