At 1920 x 1080 ....the 6xx series in a heartbeat....At larger resolutions its a closer call but I'd generally lean 7970.
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 PE gets 851 fps for $300 at a cost of $0.35 per frame
7870 gets 701 fps for $280 at a cost of $0.40 per frame
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
7950 gets 746 fps for $330 at a cost of $0.44 per frame
7970 DCII gets 924 fps for $415 at a cost of $0.45 per frame
680 DCII gets 1077 fps for $520 at a cost of $0.48 per frame
7970 gets 872 fps for $420 at a cost of $0.48 per frame
7970 GHz gets 952 fps for $470 at a cost of $0.49 per frame
680 gets 989 fps for $500 at a cost of $0.51 per frame
I use these more than anything else @ 1920 x 1200/1080:
http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_670_Direct_Cu_II/33.html
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121637