wouldnt get any less than a gtx460 or ati 6850. What are the rest of your system specs? There are other things you need to take into account to get 50+ fps and you need a decent power supply to run a decent vid card.
From what I've seen, a 6770(5770) is able to run at 40+ on high settings at 1980x1080p. With no AA of course. And to be honest, the game looks gorgeous on low. Looks better than most FPS today. On medium you wouldnt hit below +45, which is still very smooth. So it's safe to say you wouldnt hit below 50FPS on low at 1980x1080.