[Fingers]

I've done a bit of research on the subject and as it turns out ATI basically hasn't released fully functional drivers for Vista yet. Among other things OpenGL is not supported at all by the current (beta) version of ATI Catalyst Vista, and Microsoft's own OpenGL implementation doesn't work as advertised (it doesn't use 3D acceleration at all). Some if not all NVidia cards are similarly affected. Ironically, integrated graphics chips from Intel apparently have fully working drivers already.

I would not recommend getting a Vista machine at this point if you're interested in gaming. Besides most independent games, OpenGL is necessary to run games based on Quake3 or Doom3 engines and many others; typically, whenever a game is released on both PC and Mac it uses OpenGL because the standard exists on both platforms.