Basically, get the fastest card you can afford. If you don't want a multi GPU solution, it's the GTX 480. If you want something quieter, cooler, cheaper and drawing less power (even though, I find all the heat/noise criticism of the 480 exaggerated), go for a 5870, preferably with 2 gigs of VRAM. A shelf lower would be 470 or 5850.
Don't let people misguide you by claiming that a midrange card "slaughters" full HD resolution. It's simply not true. Good luck playing the most demanding games on a midrange card in 1920*1080 with max details and 4/8*AA. Also, the statement "1 gig of VRAM is more than enough for any single monitor gaming needs" is also misleading. Look here:
http://www.pcgameshardware.de/aid,7...t-Was-bringen-2048-MiB-VRAM/Grafikkarte/Test/
I have a single GTX 480 for my 1920*1200 gaming, and let me tell you, there are enough examples of games where a faster GPU setup would be most welcome. I simply wanted to return to single GPU gaming so I grabbed the fastest chip available (and I'm happy with the card, mind you). Otherwise, I'd have bought 480 SLI for 1920*1200. Overkill? Not if you want really smooth fps @ max settings in games like Crysis, Metro, JC2, etc.