If playing at 1920 x 1200, the 560 Ti factory OC'd versions are just a few percentage points off the 570 but much cheaper.
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:
$ 360.00 6970 (526/825) $ 0.68 - $ 0.87
$ 220.00 560 Ti - 900 Mhz (495/862) $ 0.44 - $ 0.51
$ 320.00 570 (524/873) $ 0.61 - $ 0.73
$ 480.00 580 (616/953) $ 0.78 - $ 1.01
As you can see, the 862 fps total for the twin 900 MHz 560 Ti's at $440 is just 11 fps off the twin 570's at $640 or 6970's at $720.
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&IsNodeId=1&srchInDesc=Ti&Description=560%20GTX%20900Mhz&page=1&bop=And&Order=PRICE&PageSize=20
At 2560 x 1600, I'd look at the 2GB 6970's