Big enough to make the additional cost worthwhile
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:
$ 140.00 6850 (371/634) $ 0.38 - $ 0.44
$ 160.00 6870 (434/701) $ 0.37 - $ 0.46
As you can see, the 6870 has a lower cost per frame at 37 cents....not quite the purported double frame rate as reported above but it is 17% faster for just a 14% increase in cost which is a sound return on investment in my book.