Radeons are cheaper but they aren't nearly as fast as Nvidia cards.
I'd recommend a GTX 460 at lowest. Or perhaps if you can afford it go for a GTX 550. EVGA sells superclocked, 2gb 550s at my best buy for $200, and there's a markup at retail so check around Newegg, Tigerdirect, ebay etc to try and find one. You might even be able to get a used 550ti for around the same price.