CPU? Any over clocking?
The 770 is a 225W card, so two in SLI will need 450W. The CPU will be anywhere from 75-125W. Rest of system is normally up to 50W give or take. So assuming a normal system and all stock, up to 625W is needed. Any good 750W will be fine. Go higher then that as you need for OCing, etc.
Edited for math. And Coolcole01, if you think those cards need great power, what do you think the AMD 290x needs?