HD7950, no contest.
HD7850 1GB crossfire would have the raw grunt advantage, but simply wouldn't have enough VRAM to make use of the power.
I would also assume (I havn't run 1080p for a while) that settings that would keep you below the 1GB VRAM limit would be more than playable on a single HD7850 1GB anyway.
The same problem plagued the GTX570.
Now in Surround
BF3 (Origin) - Xaser04