Here's what I would do.
1) Remove every PCI card, disconnect any unneccessary USB devices, and remove all but one RAM module, then try to boot.
--- If it doesn't boot, try another RAM module in its place.
2) If it boots, I would shut down, put another RAM module in, and then boot.
3) Repeat step 2 if have more than two RAM modules.
4) If it still boots, I would shut down, insert a PCI card, and boot.
5) Repeat step 4 if you have more than one PCI device.
6) If it still boots, add a USB device, and boot.
7) Repeat step 6 if you have more than one USB device.
That helps isolate it to a potential problem device. But it might also be the motherboard or potentially the PSU. CPU is unlikely, but a possibility. If you go through all those steps and still have the problem or you can't get past step one, then you're probably going to want to see if you can try another known working PSU that will support the load you have there. If you can't do that... well you're looking at probably having to get a new motherboard (and with it a CPU, RAM, and videocard since all this stuff has probably changed).