Q: Will I need a DX10 card to play Halo 2 for the PC?
A: No, but it'll probably look better. Halo 2 for PC will run on a DX9 card.
This is from: http://halo2pc.org/misc/halo2-pc-faq
If this site is correct, then although you may require vista for Halo 2 - PC, you only need a dx9 card.
This doesn't sound right.
.....Well, on another page they say a dx9 card will run the game. Why would ms insist on vista an not on dx10?