Gtx1070 sli recommended psu is 700w. 980ti is 850w. So unless there is strong consideration for sli on the 980ti, best bet is still a good 750w but use a 1070 instead of a 980ti. Also to consider is the latest DX12 installment which uses upgraded software that's available only for the 1000 series nvidia and GCN 1.1 amd cards and not available in 900 series or earlier cards. It was just released for the new Tomb Raider, and makes a visible difference.
Unfortunately i already purchased a 980ti way before the Pascal architecture was revealed
(I know i've been dreading over it)
Am thinking i'll probably just purchase the 1080ti when it comes out and have a multi-GPU with the 980ti set up to my rig since there has been news about support for non identical gpu cross setups.
Guess will go for 850w then
since there shouldnt be any consequences for having a higher wattage psu for what's needed. Atleast therell be room for overclocking