IMac G5 won't start up - logic board or power supply?

My oldest son killed my iMac G5. Apparently he'd taken to sneaking downstairs in the middle of the night and watching videos on YouTube. One night, during his viewing session, the iMac threw up a kernel panic and the fan came on. My son tried to turn off the iMac but couldn't find the switch. So *he went to bed and left the iMac with the fan on full-blast.* When I came downstairs to go to work, 6 hours later, I turned off the machine, waited for a minute or so, and then attempted to turn it on.
The iMac is stone dead. Will not turn on.
I'm deathly afraid the logic board is cooked. Those things are damned expensive. I held off on doing anything for a while, but then I was talking offhand with a Genius at a local Apple store. He said it might be as simple as a power supply. Which would be great - $125 is a lot cheaper than $800, and I could do this repair myself. But I don't want to sink $125 into the machine unless I have a pretty good idea that this will, in fact, fix it.
Anyone else had this problem? Did it turn out to be the logic board, or the power supply, or something else entirely?

First off, the kid is grounded - second, which model do you have? Have you tried resetting the SMU? Resetting the SMU can resolve some computer issues such as not starting up, not displaying video, sleep issues, fan noise issues, and so on. If your computer still exhibits these types of issues even after you've restarted the computer, try resetting the SMU. To reset the SMU on one of these iMacs:
1. Turn off the computer by choosing Shut Down from the Apple menu, or by holding the power button until the computer turns off.
2. Unplug all cables from the computer, including the power cord.
3. Wait 10 seconds.
4. Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.
5. Let go of the power button.
6. Press the power button once more to start up your iMac.
Let us know if this helps,

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