64-bit XP has gotten very little love from developers, and drivers are not readily available for most games. In 64-bit Vista, I have only been unable to get one game to work, and that was Theme Hospital, and that was because it wouldn't read my newer sound drivers. If you have and are sticking with a DX9 only video card, I don't see a reason to go to Vista.
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P5N32-E SLI 8800GT Q6600
2gigs Mushkin Redline @DDR2 800, 4-4-4-10