Anything, also games, should just stick to standards. Real standards with published specs. Like OpenGL. So then it is hopefully clear how many features your chip can do in hardware and what needs to be done by CPU.
Okay, I see that DX tells what can (hopefully) actually be done in HW. And maybe MS offered tools for easier programming to the game devs. But even then if a game needs DX9 for best display then it should work with any DX9 card absolutely independent of chip/card brand.
edit: Oh, damn nerdish behavior. Posting on xmas here. Even though I "have a life".
It's christmas so merry xmas to you folks!