I do not believe War Hammer is a DX10(.1) game. Saying that, it wont matter or not if you have a DX10(.1) card because you are running in DX9. But, newer cards will give better picture quality and ATi has been known to have better color representation.
I agree with efx92. Get Vista anyway. You may not like it at first, but understand that it is a new OS and not XP. If you go into it with an open mind and ready to learn a new OS, you will love it. If you want it to be XP v2 then you wont like it. Vista is overall better than XP.
There are a few games coming out that will be DX10(.1) compliant. Other than Crysis, Assassins Creed, Bioshock, UT3 that are already out; Star Craft 2, Diablo 2, Guild Wars 2, Spore, and more are coming out that will be DX10(.1) ready as well. So getting a DX10(.1) card is well worth it. For $900 I would get two Radeon HD4870 and cross fire them if you can. If not, wait for the HD4870x2 (that will run about $450 and smash the Nvidia GTX280).