How much do you have? Are you gaming? Take a look at some in your price range, and check www.tomshardware.com and/or www.anandtech.com for benchtests of the games.
(Btw. Nvidia is prefered atm. due to Pixel Shader Model 3 support, that the next gen games will use. ATI will be realising their X900/R520 soon, which will support PSM3.)