Run Down:
The 680 has on-paper specs superior to those of the 7970 with bandwidth being handed to the 7970 because of it's 384-bit bus.
In benchmarks it appears even the 670 bests the 7970. Which in turn is practically on par with the +100 USD GTX 680
Bottom line: Neither, a 670 4GB (particularly the EVGA FTW and ASUS Direct CUII) , is the best bang for the buck with the 680s price also being outrageous.
The extra 2GB in the 7970 will make 0 difference and only adds $$$$$$$$$$. .AMD has been on a good driver spree recently so you might want to consider a 3GB 7970.
PS: If you are going for such jaw dropping specs you might want to invest in a Liquid CPU cooler like the pre-filled H100
On-Paper benchmarks: http://www.hwcompare.com/12351/geforce-gtx-680-vs-radeon-hd-7970/
Game Benchmarks:
http://media.bestofmicro.com/U/3/336747/original/gtx%20670%20vs%207970%201920.png
http://media.bestofmicro.com/U/1/336745/original/gtx%20670%20vs%20680%201920.png