To the OP, the best idea is probably to go with a single card 680 for now. Spend the extra $$ on a 120hz monitor. If youre only playig a 60hz it doesnt make a damn bit of difference which card you pick. Later on when games like Crysis 3 and the Unreal 4 engine come out you can just buy another one for less than than it would cost now.
There is one big factor thats been overlooked in this thread so far and thats the fact that GK104 cards are still very new and should start seeing decent performance increases over time. Right now the 7970 can trade blows with the GTX 680 when the clocks are matched up. Pretty soon a steady stream of Nvidia drivers will start to change that. Even my GTX460's that are close to 2 years old have seen improvements with recent driver updates. Over the last 6 months games like Skyrim have improved nearly 25% just from driver updates. Crysis 2, BF3 have seen noticeable improvements as well. My vote goes for Nvidia.