750-850w is overkill but the 780 Ti is a power hog and I wouldn't overclock it and the CPU with less than a 600w unit myself.
Yeah but it's still best to buy a power supply that'll give you a good amount more, about 2X, than the power draw. I'd personally say somewhere in the 1.75X field since max power draw, where all components are at 100% usage, is rare. But if you want it to save electrical bill money, I personally think it's worth the extra money for an ideal wattage as well as efficiency rating.