It probably depends on what your Power supply you use. Generally, 550w is the requirement of the 770.
I agree. The 770 requires around 190W max (16A rounded up on the 12V rail). If you get the right 500W PSU, it will easily run a system with a 770. Seasonic, XFX (all built by Seasonic), and Corsair are companies that design PSUs with enough available amps and watts on the 12V for a system with a 770 in the 500W range..