Im on a 560Ti, i dont know about the BF3 engine, but except metro, i run everything on highest settings and i dont expirience any lag (bare in mine im using a q6600 on 3 ghz).
Metro i run high 1650x1080 high, tesselation on, DoF off 4AA.
After the game comes you we will still need to wait for game patches and driver optimizations to see the real performance in the game from the current gpus.