You decide:
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:
$ 210.00 6950 (479/751) $ 0.44 - $ 0.56
$ 260.00 6950 Frozr OC (484/759) $ 0.54 - $ 0.69
$ 205.00 560 Ti (455/792) $ 0.45 - $ 0.52
$ 360.00 6970 (526/825) $ 0.68 - $ 0.87
$ 205.00 560 Ti - 900 Mhz (495/862) $ 0.41 - $ 0.48
$ 320.00 570 (524/873) $ 0.61 - $ 0.73
$ 480.00 580 (616/953) $ 0.78 - $ 1.01
-Dual 6950's ($420) will garner you 759 fps in the test suite at a cost of 56 cents per frame
-One 580 ($500) will garner you 616 fps in the test suite at a cost of 78 cents per frame
-Dual 900 MHz 560 Ti's ($410) will garner you 862 fps in the test suite at a cost of 48 cents per frame
The last is the lowest cost and highest performance:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125363
I don't have experience with the Gigabyte model but I have gotten 8 Asus ones to 1000 MHz, only one req'd voltage adjustments.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121425
Here's a review where they OC'd it to 1017
http://www.overclockersclub.com/reviews/nvidia_asus_gtx560ti/4.htm