A pair of 6870's needs 20 - 25 amps. Figure another 10 - 12 amps for an OC'd CPU and another 80 - 100 watts for the rest of the system. That is somewhere between 440 and 550 watts.
Your GX750, if properly working, is more than adequate to power your XF'd system.
Thank you so much, i was thinking on buy a powerful PSU but now i can figure out this is a RAM issue.
I don't know for sure what it could be, i'd run windows memory diagnostic and it says my ram it's OK.
so, what else could be? is there a program or something to check gpu integrity? i mean, to check if there is something wrong with any of the two video cards?