Going with reasonable budget I'd recommend one 1024mb GTX460 now and second card later. Also pick up 850W good quality PSU, like Corsair HX for example.
ATI is not really recommended because their current cards suck at DX11 tricks, but Radeon 5830 is an alternative too depending on price. 5830 and 460 are nearly equal in games that do not use PhysX or tesselation, but Nvidia is clearly better for games that do use those. Radeon 5850 is slightly better but not worth it unless you can find really good deal on it. Factory OC'd 1024mb GTX460 is equal to 5850 and lot cheaper.