Well... it really comes down to how hard you'll be pushing these cards. If you plan to run them at 100% TDP, do yourself a favor and get TWO PSU's, running that PSU near 100% Full load will make its efficiency go down a lot(Lose more money than save). However if you undervolt these cards to about 45%(What I'd do) you'd be pretty good wattage wise(1080 ti's going for 124watt per card, 1070's probably 100watts) should be alright. Just remember going full TDP is never a good idea if you plan to play it safe and long term.(Unless you want to change out parts a lot then by all means go nuts)