The 6870 is the best bang for the buck followed by the factory OC'd 560 Ti.
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:
$ 170.00 6870 (434/701) $ 0.39 - $ 0.49
$ 220.00 6950 (479/751) $ 0.46 - $ 0.59
$ 240.00 6950 Frozr OC (484/759) $ 0.50 - $ 0.63
$ 205.00 560 Ti (455/792) $ 0.45 - $ 0.52
$ 320.00 6970 (526/825) $ 0.61 - $ 0.78
$ 215.00 560 Ti - 900 Mhz (495/862) $ 0.43 - $ 0.50
$ 340.00 570 (524/873) $ 0.65 - $ 0.78
$ 500.00 580 (616/953) $ 0.81 - $ 1.05