Boot Issues (Advanced Users Only Please)

So here's the issue. I have a 20" iMac G5 iSight that came to me with a bad hard drive. It was showing the folder with a question mark so I pulled the hard drive, ran a diagnostic and the drive started clicking before it even finished.

I threw another drive I had in that had tested good, put in my Tiger OSX install disc and held down C, but it doesn't want to boot from from the CD. It tried booting from the HDD but since it has an old Windows install on it, it came up with a disk read error and some odd artifacts on the screen. I thought maybe the fact that it was formatted for Windows might be some sort of issue, so I pulled the HDD out of a 17" iMac G5 iSight, but that just gives me the flashing folder with a question mark icon and doesn't let me boot from my CD either. I assume that since these are from the same series with very similar hardware I should be able to switch the HDD and boot from it without having to reinstall Mac OS, at least I've always been able to do that with similar MacBooks.

I tried switching out the CD drive with a known working one as well, but no dice. Cleared the PRAM several times, didn't help. I held down the option key on power up to enter the boot selection, but all that happens is the screen stays white and a black cursor pops up that I can move around, but nothing else at all comes up. I've tried this with both hard drives and I get the same issue where only the cursor comes up.

Also, if it makes a difference I'm using an older G3 series keyboard with it. I thought maybe that why I couldn't boot from CD at first, but it lets me clear the PRAM just fine. So now I'm at a loss. I've never encountered an issue using the option-boot, so that confuses me the most. I'm thinking it's possible the motherboard is going, but I've never seen these sort of issues with a bad mobo.

Anyway, I'm open to any and all ideas.