Its too bad the motherboard doesnt have integrated GPU to fall back on for a lower level of troubleshooting. I have doubts its the power supply, and thinking its main board.
**Hopefully if its a video card that requires 12V Molex power, that the power is given to the videocard and not just relying on the PCI-E power. That is one way i could see a video card post beep occuring and not have it the power supply or motherboard.
What video card are you running... got us both curious now?