Question about Corsair HX750i

yes, the 1080ti needs about 300w and the 8700k about 150-190 with oc, which means that a 500w power supply would theoretically suffice, but since power supplies are most efficient at about half the power, you'd be well advised to go with 750w. since the 8700k will probably often not be fully utilised.

Yes, 750w is quite enough. I've had one for years and it runs like clockwork.