I want answere

That is a hard question to answer right now.
A long time ago when dial up was at 36kps a sec was the best and the bleeding edge on Internet speed I use to have to download a demo of a game before I bought it because some games would work on some GPU's and not others.

No days most every game will will use Direct X ( even though I still think open GL was and can still be a good standard to use if they worked on it ).

A most games still use DX9, some use DX10 a few let you use DX11.
DX11 has some promise of some future game to come too.

Then you also have a few games that can look better with Physx X ( Nvidia tech were you have two like cards SLI or a older card dedicated as the physx X card).

Then there a some games that have better FPS if you use a ATI GPU and Vice Versa better if you use Nvidia.
If you want to be ready for some of the future games buy a ATI type GPU 5850 or 5870 being that they support DX 11 now and Nvidia doesn't
If the eye candy of future games isn't what you are looking for I would suggest a ATI 4850 or 4890 .