Go for the single gtx 670. According to benchmarks it performs better than sli 560's, and it uses less power, creates less heat and is quieter. On top of that if you want to you can buy a second for sli down the road
That,s an easy decision, the 670, I would suggest purchasing one of the models that is on a full 680 size board, the ASUS DCII's look like a great card, if they perform anything like the 680DC2T you will be very satisfied.
