I doubt this is a bad cpu. To toast the CPU you'll need to increase the voltage. When you overclocked it (I assume you just changed the jumper settings) you didn't change the voltage to the CPU core, therefore, it won't overheat. You are just stressing it a bit, but that definitely won't heat the CPU enough to damage it.

I have a G3 B&W with a 300Mhz processor upgraded to 450Mhz, and I never had a problem with it. I also have a PowerMac G4 PCI (Yikes) with a 400Mhz G4 running at 450Mhz. One day it just stopped booting. I tried everything with no luck. I left it unplugged during the night (I removed the battery). Next day, I started it up, zap the PRAM and it booted fine, something I had already tried the night before like one thousand times. So keep trying and it may come back. The fact that it makes the boot sound is a good sign.

PS: Does it boot to open firmware? Cmd+Alt+O+F (I think) Or, at least, do the fans start spinning?

The mobo also has a few confirmation LED's, look if they're all on. You can try booting without any RAM installed to see if it does the 4 beep sound warning there's no RAM installed.