What exactly is the PSU? 750w doesn't matter if it's a 10 amp 12v rail,
And like Shank said, check the plugs from the PSU to GPU. Unplug the 270x and pull it from the MB, then put it all back together again and see if that helps. May not be seated right.
I believe that the 270x uses a 6 pin and 6+2. If it does then switch them around. If it works then that should let you know if the PSU is the issue.
Also if you can try the GPU in another computer. Friends or something. Your goal is to try the different components and see if the PSU, MB, or GPU is the issue. Older MBs may need a BIOS update before they can run newer cards, especially AMD cards.