Depends on your display resolution-
For 1920x1080 resolution, the gtx 670 would own the hd7970.
For 2560x1600 resolution/ multi display gaming, the hd7970 oc would perform better.
many might argue the hd7970 dual x would beat the gtx 670 with high amount of overclocking, I do not believe thats the case. The Asus gtx 670 direct cu II Top has an automatic boost clock frequency of 1300mhz+ it even beats an overclocked gtx680. Even a highly overclocked hd7970 would struggle to match that power at 1920x1080. At higher resolution, however, it's a different story.
i got samsund led 23 inch monitor (1920x1080)