I bought the Corsair AX850W but now I am afraid it is not enough. I used Antec´s Power Supply Calculator and if I put my system on overclock and consider a (recommended) 20% loss from the PSU's capacitor aging, it tells me I should be getting a 1000W PSU!
20% capacitor aging isnt realistic for the AX850. First, kit guru was able to pull more than 1000W from it during the overload portion of their review before it safely shut itself down, second, they use 105C rated capacitors which age much slower than an 80C rated capacitor does at the temps a PSU runs at so it ages much more gracefully.
Capacitor aging on those calculators leads to it grossly over estimating power draws. If we add up the stuff in your system by hand, about 260W for a GTX 580, about 130W for an 980X, about 20W for ram, 10W for SSD(worst case) and about 50W for Motherboard and fans only puts you at 730W, which leaves you a nice amount of room to OC before you come near the limit of the power supply.