i've had both of them and there both very good and the can take eg: Left 4 Dead 1 and 2 or crysis/-warhead on MAX these two graphics card is a must, bcuz the geforce works with Nvidia 3D and Ati is good bcuz the performance is outrageous.
i recommend 2 very good and cheap graphics card:
1. http://www.newegg.com/Product/Product.aspx?Item=N82E16814150447
2. http://www.newegg.com/Product/Product.aspx?Item=N82E16814143211