When vendors/manufacturers have recommended PSU watttage for GPU's they take into account people who have bought chinnese bombers/low brand PSU's.
For your rig a quality Gold rated 750W psu will be more than enough to run your set up.Best choice at level is the Evga Supernova G2 750w or SF leadex Gold 750w.Even if you Oc'ed both cards and your CPU you would still only be drawing around 600-650w at load
An 850w PSU would be excessive,1000w would be overkill
Guru3D, are vendors/manufacturersLOL.
http://www.guru3d.com/articles-pages/nvidia-geforce-gtx-970-and-980-reference-review,7.html
So if i wanna overclock better take 750w?