While I agree the 770 will max everything out right now already, the 780, having more power and more VRAM, will last much longer into the future than the 770. If you are fine with spending the 350 quid on the 780, I would go for that, since it will last you longer. Otherwise, the 770 is a great choice
As long as the power supply is of good quality, 650w is more than enough
A system running a 780 or 770 will use around 350w MAX at peak load.