As already stated above you need a 6670 or 6650 for hybrid CrossFire.
If you want to just use the 7750, disable the onboard GPU and select PCI-E for boot in the bios. Then, plug the DVI/HDMI into the 7750 and boot up.
i already made this...i understand i need 6series for crossfire. but the system don't boot up. i see bios info, i see that windows logo with that circle loading and when the windows must showing your picture with your name etc etc... i dont see nothing black screen blinking to black and gray..or something like this maybe something with the bios or with the drivers?