2 gtx 570s or one gtx 680
I dont know how the GTX scale. 2 gtx 570s cost 550 and one 680 cost 500 (obviously)
Plus the cost of a 750w psu vs. 550w, and a more expensive sli capable motherboard, and better case cooling.
I think the sli GTX570 will bench higher, but you will get full performance on all games with the GTX680; some games do not play well with sli.
If you will be using a single monitor, then I vote for the single GTX680, if you can find one.