HD 7970GHz edition is currently the strongest single CPU card on the market with the 12.7 drivers, I believe? However, the GTX 680 has the upper hand with less power consumption, less noise, and more overclocking headroom. However, that all depends on what aftermarket cooling solution you choose.
As far as the VRAM goes, I wouldn't see why 3GB or 4GB would be necessary at 1080p resolution, for the the sake of it, the 7970 support a wider 384-bit memory interface compared to the 680's 256-bit. That allows the 7970 to achieve faster performance even though it has less VRAM. So lets look at the tally
Reviews - HD 7970
VRAM - Tie - GTX 680 has one more GB, but 7970 has a wider memory interface.
Temps - GTX 680 (Subjcet to Change)
Noise - GTX 680 (Subject to Change)
Overclocking - Subject to change.
It's a pretty even split, to be honest, the truth lies in the aftermarket solution you choose.