Anything from the following will have full Windows 7 driver support,
1. AMD/ATI - HD 2xxx, HD 3xxx, HD 4xxx
2. Nvidia - Geforce 6xxx, 7xxx
The fun part with those cards is that in a lot of cases they cost more than the computer they would go into is worth. If you can find one used it helps, but the prices for new make it a less than desirable "upgrade".