Seriously, I could see a 850 watt quality PSU but I didn't realize that much power draw.
A 7970 stock under real world moderate load (not stress testing) without any other components take 214-34W = 180 Watts.
The TDP is 250 watts for a stock card for stress testing etc purposes.
A 7970 CF stock without any other components with moderate-high (not stress testing, not max usage) takes 411-34W = 377W
But if you consider TDP point of view then they take 200-250 watts each max, or 400-500 watts both.
But once you go in the high OC range, that is past CCC, you should be ready for 250+ watts per card. So realistically with some scope, 300X2 = ~ 600 watts just for the system.
Without the GPU, a high end system will take:
2600k at 4.6 1.4V
Asus mobo
4 DDR3 RAM at stock
1 Blu ray
1 SSD
1 HDD
H100
4 USB
1 sound card
2 x 120 mm fans
misc
these take about 359W at full load.
So that means 1000W is what you need from a brand like Seasonic and at least 1200+ watts is recommended for some room, capacitor aging etc.
However, this is at full load, stress testing load.
Under real world situations you will probably stay within 600-800 watts or so.
However, no point in being so finicky about not spending an additional $100 for a PSU which can use everything at 100% load oced comfortably, especially when you are spending a few grands anyway