That is the exact problem. The Corsair CX750 isn't a quality unit and I wouldn't be suprised if it doesn't deliver the full 750w under full load. I think it more likely will deliver around 720. That's why you shouldn't try it.
Edit:No headroom at all with the FX-8370. Your power supply will remain in continued stress and may aswell lead to reduced life length of the PSU.
Buy a newer single graphics card. Crossfire with old GPU's is a no no anyways.
But I dont have much money. Where another new 6970 would cost $90, compared to a $250 card that would equal the power of the Crossfired 6970s... ( I mean crossfired 6970s equal about 1 GTX 770. http://www.newegg.com/Product/Product.aspx?Item=N82E16814487108
It said Minimum PSU Wattage should be 682, and their recommended was 732. Now I know this doesn't leave any room to overclock the CPU, but I don't plan to.
That is the exact problem. The Corsair CX750 isn't a quality unit and I wouldn't be suprised if it doesn't deliver the full 750w under full load. I think it more likely will deliver around 720. That's why you shouldn't try it.