Have you tested the power supply? If you don't have a tester I would suggest getting the Antec Digital Power Supply Tester
. I have one and I love how it does not just say good or bad but it also gives you the exact output voltage that you are getting. You can pick one up at your local Best Buy. I did some searching online and could not find it for any less then that but when I searched from "Power Tester" on eBay it came up with some other brands that looked like they did the same digital output display as the Antec one.
As for if it the power supply is not the problem. I would suggest pulling out everything except the CPU and it's heat sink/fan and see if it gives you a beep code for not having RAM. Then if it does put one stick of RAM back in. You should also see if you can dig up an older video card. Even if it is regular PCI. About the only other thing I can see is if it stays powered up but does nothing you should look into getting a PCI post card and see if that tells you anything.