I can't think of a single valid reason to buy a 680 over a 670 for the price difference. The difference in performance is so small that its just silly to blow an extra $100 for something you'll never likely see except in a benchmark.I have two MSI 670 PE here my results with just one card My clocks/results on the 670 PE were:
Core: 1316 MHz
Memory: 7406 MHz
Core: 1290 MHz
Memory: 7204 Mhz
Fps: 51.5 if you insist on buying 680 go for Asus 680 TOP.