At 1920 x 1200.....two of these beat both the 680's and the 690. yes, you can get an OC'd 680 too but the cost per frame is significantly higher.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121637
http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_670_Direct_Cu_II/33.html
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:
660 PE gets 851 fps for $300 at a cost of $0.35 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
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
For bigger resolutions on single monitor, I tend to lean to AMD cards .... on multi moniutors, look at some of the graphs here:
http://uk.hardware.info/reviews/3026/nvidia-geforce-gtx-660-ti-review---3-way-sli-included
Not necessarily that particular article but that's one of the latest ones so has the most current data.