A 850W can run a crossfire of 290X, but if you have them watercooled and OC the CPUs and GPUs, then run something insane that'll tax all 3 processors, then yeah, you need a beefier power supply.
TweakTown was saying their i7 4770K and dual Sapphire Tri-X crossfire ran at 1050W.
Just yikes.
I've...