Power Supply bad?

[Sil23]

Recently I went to turn my comp on and had a no signal to the monitor, after trying again and again to get it to recognize it (never had an issue before) I decided maybe the mobo was bad. (Biostar socket 754, AMD 3200+, onboard vid card, nothing real special) So I tracked down a new socket 754 with a sempron 3000 chip, replaced the chip, throw in a 512 vid card and hooked everything up, it all worked just fine, save the face the USB ports and the Ethernet port weren't working (light would flash on the Ethernet port and then flash off). A normal keyboard worked, but never the USB connections, so while trying to figure that out, I once again started to get the no signal to the monitor issue, still get it after unplugging the vid card and trying to use the onboard vid card, nothing. I've tried 2 monitor cables and 3 different monitors (I know they all work) the PSU is a 450W, I had a 500W but the fan stopped : / Is there any way to tell for sure if its the PSU? Just trying to tell for sure before I go buy one. Thank you for any help : )