#1) Your video card won't be useless for new games. Nvidia will support you with new drivers for DirectX10 support.
#2) XP will always be upgradable to Vista. If you get XP now, you can get a cheaper Vista upgrade later, rather than paying the full blown Vista price.
#3) If you're a gamer, your processor of choice would be from the Athlon 64 line-up, with their 64-bit capabilities, they will support Vista very well.
|Intel Q6600 | 2GB Crucial Ballistix Tracers | XFX 8800GTS 512 | Gigabyte P35-DS3L | Antec 900 | Antec Neo 500 HE |