My suggestion is to try just one GTX770 and see how you do.
The GTX770 is a good card, and I suspect that might be all you need.
You can add a second later if you really need it.
I have my doubts about 4gb of vram. It is there because it sells.
Here is one analysis using GTX680, a similar card.
The net is minimal benefit.
http://www.pugetsystems.com/labs/articles/Video-Card-Performance-2GB-vs-4GB-Memory-154/