The CM-x PSU quality depends on which version. The old ones weren't up to snuff, but it's gone through many revisions with the same branding. I wish corsair would just label them by Version or Year of release. But the only way you can tell is by the label, or disassembling them. But if it's over 5 years old, I would say it's no longer good to use on the GPU. It would be extremely borderline when it was new. They have a tendency to degrade over time.
Could the GPU be bad? Yep. But the power supply is questionable at best and the place I would start.
PEG is short for PCI External Graphics. You can tell the BIOS to initialize motherboard or PCIe graphics first. Sometimes in rare cases that makes a difference as long vBIOS initialization will make it appear like there isn't a graphics card there unless you force it to boot first. But if your system isn't booting at all, even with the built in graphcis header on the motherboard (with the 280x plugged in), I would say it's a PSU issue as the first guess.