In terms of overall performance, they are equal. A few FPS makes no difference. Note in the chart below we are looking at vanilla 7970 vs. vanilla 670. (The Ghz edition is not available for sale anywhere and would be more expensive.) If we are talking about the reference 7970, the noise level is very significantly louder, particularly with all of these suggestions to overclock and pump up the voltage. The GTX 670 also overclocks by a lot, easily into the 1200 mhz range and it comes with Turbo Boost to get even higher overclocks. The gains for either card will depend again on the game being played.
Exterrestrial, I encourage you to learn about PhysX and some of the other extras that have been listed. AMD simply does not have anything comparable. Then once you have all the facts, make your decision, which is going to come down to:
"If I overclock a 7970, is the minimal performance advantage enough to justify it over a GTX 670 that also overclocks, runs better on BF3, is quieter, uses less power, and comes with PhysX, Adaptive VSync, FXAA, forced Ambient Occlusion, Transparency Supersampling, and better driver support. A few FPS either way with these cards simply makes no difference, but the bigger picture does.
Adaptive VSync review
http://www.hardocp.com/article/2012/04/16/nvidia_adaptive_vsync_technology_review/
http://www.techpowerup.com/reviews/AMD/HD_7970_GHz_Edition/28.html
For fan noise, 6 Db is significant. You can see by looking at the Ghz numbers what a little overclocking does to the noise levels.
http://www.techpowerup.com/reviews/AMD/HD_7970_GHz_Edition/27.html