The graphics card is the most important component for any gaming build.
So you should look at ATI Radeon HD4850, HD4870, HD5750, HD5770, 5850 or 5870 depending on your budget.
If you like nVidia card more get a Geforce 9600GT, 9800GT, GTS250, GTX260, GTX275, GTX280, GTX285 or GTX295 also it all...