I plan on buying 2 6970s and using a Corsair AX1200 or Cooler Master Silent Pro Gold 1200w. I know these are a little pricey and high power, but I don't want to be pushing the limits of the PSU, I plan on keeping this PC for a long time and want the power under load to be about 60% so that I don't risk blowing it, hence why I went for 1200w and not 1000 or 900.
My question is what is the average OC of a 980x under Corsair H70 (or equivalent) with UD7 (or equivalent) while still being within safe voltage limits? (To my knowledge this is 1.4 vcore 1.4 qpi 1.65 dram)
Your about right with those voltages, I wouldn't let the temps exceed 80C under load. I would imagine you'd get to 4Ghz easily, whether or not you'd get higher than that depends on how lucky you get with your CPU (some overclock better than others.)
I've got to 4.3Ghz with 1.360Vcore and 1.355V QPI, and my Core i7 920 isn't a good overclocker, my 920 stock speed is 2.67Ghz vs the 980X's 3.33Ghz, however the 980X has 6 cores so it may be harder to overclock.