phhhttttt. a 500w psu is enough for this build, even for a little overclocking. But if you want to do a lot of overclocking then 550 or higher is preferable. If you want to run another 770 in SLI in the future then a 750w psu would be preferable
Based on the following measurements
http://www.tomshardware.com/reviews/build-your-own-gaming-pc-core-i5,3708-17.html