A pair of 6950's will pull about 24 amps, peaking to 28 amps. Figure 10 amps for an OC'd CPU. That is around 450 watts.
Figure 40 watts for a couple of hard drives and an optical and another 80 - 120 watts for the motherboard and RAM.
That is around 600 watts. A Corsair TX750 will be operating at around 80% of rated power at peak computer loads. I'd prefer a somewhat larger power supply, but a good 750 watt PSU is perfectly adequate.
2 5850, and an HX650 (50 amp rail):
5850's are pretty efficient. A pair of them will pull about 20 - 24 amps. CPU - 10 amps. That's around 400 watts. Add another 100 - 120 watts or so for the rest of the system and you are up around 520 watts for about the same load factor.
I am running both of my 560ti's an oc'ed 2600k perfectly fine on my antec high current 620w. I asked a similar question a few months back and people said it would perfectly fine to run 2 6950's or 560ti's on a efficient 620psu. You should be fine.