well the most is price range and very great is radeon 9800 its great! i have 9600 and im very pleased
if you want to play FPS-games (like HL battlefield and things like that u should buy a ati radeon:.. if you want to play games like warcraft the buy a geforce... (and remember to buy a Sli ready card if u use PCI express lots.. i have 2x geforce 6800ULTRA in my computer.. thats nice..
i'd have to say ATI Radeon, had a Nvidia Ge force 4 MX440 in my comp and have had bad driver problems (have to use drivers from last year or my comp reboots when i am playing a game)