I would go with GTX 670, if you play at 1920x1080 then 3 GB you won't use at all. Actually 2 GB is more than enough.
GTX 670 uses less power and has a high clock effeftivness 6.008 gHz vs 5.500 GHz.
GTX 670 has boost which Means you won't have to overclock My gtx 660 boosts above 1100 mHz from 980.
GTX 670 supports Physx, FXAA and TXAA.
There is a reason AMD is dropping their prices so much.