evga gtx 670 superclocked 4gb should give you the best and most efficient performance. Gtx 690 is powerful but 2gb ram would be a limiting factor. The hd7970 ghz edition does pretty well at that resolution too. Sapphire toxic is an ideal gpu you can overclock it all they way to 1200mhz+
The 7970 GHz edition is the best card for that resolution. Both the 2GB 670 or 680 would do fine as well, if you prefer Nvidia.
A 4 GB 670 is IMO a terrible idea because the 670 is held back more by its 256-bit memory bus (a limitation on all Kepler cards) than it is by the 2GB vram amount. You can tell this because the benchmarks of the 4GB variants of the 670/680 show no improvement at high resolution over their 2GB counterparts.
Call of Duty (assuming this would be similar on MW3):