Yes it will just make it - not much room to spare.
The GTX 780 requires a minimum of 600 watts system power and to that add 250 watts which is the max that the second GTX 780 might draw. This gives you a combined total of 850 watts.
http://www.evga.com/Products/Product.aspx?pn=03G-P4-2889-KR
If it were me, I would go with a 1,000 watts PSU for two GTX 780 in SLI.