I know nvidia have a great linux driver for thier cards, and i do remember reading that ATI have also released a driver for theres. The nvidia drivers are on there website, so perhaps check on the ATI website? Sorry that i cant be any more help here, im at school so cant chase up any links for you (20mb/month limit :'()
In my opinion the nvidia cards are better on linux, because they seem to have better openGL support (there is no dX under linux
In case you didnt know, use glxgears to test your fps, it should be in the thousands... My geforce fx 5700 LE sits on around 3000fps from memory...