The 300W TDP is for PCIe specification, but it gives you an idea of maximum watt usage at stock. Two, overclocked 7970 with an overclocked CPU would probably be better suited on at least an 850W.
Intel i7 970 @ 4ghz
6x4GB DDR3 @ 1600mhz
EVGA GeForce GTX 980 Ti 6GB HYBRID @ 1290/7000
Intel 320 series 160GB
Thermaltake SMART 850W