noone really knows, the game is in pre-alpha stage of development. For whatever its worth, the trailers that theyve released were played on a GTX 580, but again we dont know what resolution or frame rates they were getting. If I had to guess I would say maybe a 6950.
Also expect the game to use at least 8 processor threads.
I think of BF3 to look and act a lot like BFBC2.Since they both use the same engine with BF3 useing a more updated version,BFBC2 use's Frostbite 1 and BF3 will use Frostbite 2.BFBC2 is very CPU demanding as much as CPU demanding so we should treat BF3 the same way.It takes a 6870 crossfire or a single 6970 to be able to play BFBC2 maxed out and stay above 50FPS and lets not forget you need to have a powerful CPU to back it up.
An O.C.'d Sandy Bridge i5 and a 6970 should be able to get 60+ FPS in BF3 but remember that we don't truely know what the game will demand.