There's about 12 posts in last 3 or 4 days on same subject
Guru3D uses the following games in their test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game @ 1920 x 1200) for the various options in parenthesis (single card / SL or CF) are tabulated below along with their cost in dollars per frame single card - CF or SLI:
-Two factory overclocked (900MHz) 560 Ti - 900 Mhz get 862 fps for $410 at a cost of $ 0.48 per frame. They can be OC'd another 10-12%
-Two 570's get 873 fps for $640 at a cost of $ 0.73 per frame. They can be OC'd another 5-7%.
-One 580 gets 616 fps for $480 at a cost of $ 0.78 per frame. They can be OC'd another 4-6%.
-Two factory overclocked 6950's get 759 fps for $480 at a cost of $ 0.63 per frame.
-Two 6970's get 825 fps for $640 at a cost of $ 0.87 per frame.