I was thinking of doing the exact same thing with 2 gtx 980 ti in sli , 850 w is pushing it but seen a lot of reviewers that done this using a 1000 w psu , how do u think of cooling those cards ?
had 2 gtx 970s in sli using a 700 w that was pushing it and gotten overheated and went to liquid coolers
it all depends on how far u want to oc etc
GeForce GTX 980ti - 38A and a 600W PSU minimum