750 watts is plenty to run 2 x 6950s in Crossfire. You can run a single 6950 just fine on 500-550w. The power supply is the most important part of your computer and you should get a good one. Corsair, Seasonic, PC Power and Cooling, XFX, Enermax, Silverstone and Antec are all good choices.
Unless that is a video rendering/editing build you might want to rethink that FX build and go for an Intel build. Because other than those two things the 8150 loses in every single case to the cheaper i5 2500.