The 690 just doesn't enter the fray as it's two down clocked single GPU's on a single PCB. Cheaper to get the single cards
Guru3D uses the following games in their test suite: Hard Reset, COD-MW2, Far Cry 2, ANNO 1404, Metro 2033, ANNO 2070, BFBC2, BF3, Crysis 2, AvP, Lost Planet 2. Total fps (summing fps in each game @ 1920 x 1200) are tabulated below along with their cost in dollars per frame @ 1920 x 1200:
670 DCII gets 999 fps for $420 at a cost of $0.42 per frame
670 gets 917 fps for $400 at a cost of $0.44 per frame
680 DCII gets 1077 fps for $520 at a cost of $0.48 per frame
680 gets 989 fps for $500 at a cost of $0.51 per frame
Two 670 DCII's get 1679 fps for $840 at a cost of $0.50 per frame
Two 670's get 1539 fps for $800 at a cost of $0.52 per frame
Two 680's get 1578 fps for $1000 at a cost of $0.63 per frame