geofelt: i currently have 2x HD 5850's
OK, just how good do you need to be?
I would imagine that a single GTX670 would perform a bit better than sli 5850 cards, and do it with ledd issues from dual cards like heat issues, microstuttering, and minimum frame rates.
If you will be using only a single monitor, I think you would see diminishing returns from sli GTX670, even with the strongest of cpu's.
Instead of spending $800 on two GTX670 cards, I think you would be better off to spend $500 on a GTX680, and the remaining $300 on a 2500K or 3570K and a Z77 motherboard.
Offset that with the sale of the GTX580 cards and the i7-920.