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:
The link you gave shows a core speed of 900 Mhz for the 6870 which is the "standard" clock rate. At least according to the data shown on that page, that card is not overclocked. The 6870 is 17% faster for a 19% speed increase.
6870 costing ya $0.36 per frame
6850 costing ya $0.35 per frame