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.

Click to expand...

Thanks for your attention, the machine don't boot to open firmware, in fact it doesn't respond to any keyboard commands (for example CMD+OPT+P+R to reset the PRAM or C to boot from CD). I'll report here exactly what the machine does looking thought the motherboard LEDs, the current machine config is: 256MB RAM PC100 on the J19 slot, 160GB on cable select mode on ATA/33 channel, DVD-RW and Zip 100 on the other channel (ATA/16 I think), processor on stock speed (350MHz with thermal compound) and ATI Rage 128 on the 66MHz PCI slot.

Starting up the Mac with everything plugged and with the PRAM battery on:

-Machine turns up

-Fans start to spin

-Startup chime loud and clear

-HD spin up

-DVD-RW turns on

-DS1 LED (ATA activity) turn on for a few seconds and turn off

-DS4 and DS5 LED (USB +5V) both off

-DS6 (Ultra ATA) turn and stays on

-DS7 and DS8 (Power indicator) both synchronized, both on

-DS9 (CPU BUS request) flash for a few seconds and turns off

-Computer stays on and no video

Starting up the computer after left everything internally (PSU, ATA, front panel, etc...) and externally disconnected and the PRAM out for 10 minutes.

-Connected everything internally

-PRAM battery back in

-Pushed CUDA button for 15 seconds

-Connected peripherals and power cable

-Machine turns up

-Fans start to spin

-Startup chime loud and clear

-HD spin up

-DVD-RW turns on

-DS1 LED (ATA activity) turn on for a few seconds and turn off

-DS4 and DS5 LED (USB +5V) both off

-DS6 (Ultra ATA) turn and stays on

-DS7 and DS8 (Power indicator) both synchronized, both on

-DS9 (CPU BUS request) flash for a few more seconds

-Mac beeps 3 times

-DS9 LED turns off

-Computer stays on and no video

I also did this test by changing the memory module and slot and left the ATA cables disconnected, but the results are the same. xx(