For three GTX 770's, you'll need 1000W or higher PSU. The 770's would be faster than two 780's in theory. But, I always add the possibility to buy another GTX 780, which would destroy the three 770's. If you need the best results right now, go with three 770's, if you want to have somehow more "future-proof" PC, go with two GTX 780, with the option to buy one or two more GTX 780's in the future.