MAX settings! That might be the problem. I run 2 HD5850's and both cards are 1GB cards, And no the Vram does not add to 2GB. But when I run BF3 maxed out, it lags, You will need to turn off AA and motion blur because this game will use more then 1GB of vram, After I did that the games runs perfect at 1080p. Also Running at high with 2x AA and the game runs good, but the 2x AA is still like 0x AA at distance.
As I said, the two 1GB cards do not add up to 2GB of vram.
Edit: Close out any monitoring program when playing BF3 and the browser, that can cause it, Also I use the Widows Gadgets, that effects my fps just a little anyway.