Get a 1000w PSU from a reliable brand. I recommend Corsair AXi series for reliable power delivery & top-notch components. Plus it will be 80+ titanium. 850w is plenty for ur system with 2 980Ti's, but if u want a 3rd, then i dunno. 1000w +, and your good for more upgrades.
I seen on Youtube that a full system with a single good GPU can pull out of the wall about 370 Watts only, but it might spike up to 400 a few seconds here n there but go back to 370'ish. For that system, 500w is good. Sure, u want over 20% on top of the recommended total power, for future upgrades, but if you don't plan on doing serious upgrades, don't over-kill it with the PSU as the efficiency curve is about on the 60% - 80% of total power used, that is where u get the most energy savings from, depending on the brand/model of the PSU.
Asus & MSi FTW (For the WiN 🙂 !!