Like others have said, for 1920x with AA I'd say grab a GTX 260 and OC it. In some titles a 512Mb card can get by with AA at that res and other times it can not. Some users here have pointed out titles like "Oblivion" with high res texture packs can rapidly use up the ram on a 512Mb card, even @ a 1680x res. I'd say don't gamble and just get the 260. A 260 overclocked to 660-700 or so should do nicely.
Below is an old post I did with my GTX 260 when I first got it. I tried to bench card killers @ 1920x like; Crysis, WiC and Lost Planet. Crysis all Very High
is the only title that could send it to unplayable FPS @ 1920x.