A thread like this gets resurrected because someone is trying to solve a problem. An iMac G5 is perfectly good for lots of things, assuming that you have contemporary software to go with it. Look up "Wirth's Law" on Wikipedia.
Sorry for feeding the troll. Now back to the actual thread...
I don't know if it's applicable to this particular generation of PowerPC, but...
I've repurposed a lot of old Apple computers ("gumdrop" iMac G4s, but that was years ago). I *always* replaced the PRAM batteries (these were 1/2 AA batteries). One thing that I saw on occasion was this sort of "just won't boot", and when I replaced the battery it was all better.
On the iMac G5, the battery is cleverly hidden:
* Put the computer face-down on a pad.
* Take the back cover off (loosen the 3 philips screws at the bottom), lift that edge up - probably the easiest disassembly Apple's ever done.
* Remove the Airport card - 1.5" square in a plastic tray. The battery is underneath.
* Replace with a new 2032 lithium (standard PC "CMOS" battery, available in hardware stores, office supply stores - everywhere).
* The spring clip is crazy tight: you'll need to pry it up and then coax the battery out of the holder.
I still reset the System Management Unit:
* Unplug everything from the computer, including power cord.
* Plug in power cord while holding down power button in back. You won't hear any sound.
* Release power button.
* Push the power button to turn it on.
(This is fairly similar to the procedure to reset all of the SMUs on modern/Intel Macs.)
I still reset the PRAM:
* Push the power button.
* Push Command-Option-P-R quickly and hold until you hear the startup chime - and keep holding until you hear it a total of three chimes. (Three times sounds like superstition, but I read it in one of the Apple support notes.)
All that said, however...
I found this thread because I just pulled a iMac G5 that had been stored for 4 years out of my garage to donate it, and it had the same problem. Even with all of the above, it still won't boot beyond the spinning cyclotron of death + crazy fans. (Holding down the mouse button will eject the CD.)
I'll keep poking at it, though.