Modded G4, 10.4, and a Grey Screen of Death

Question

Finicky just doesn't do justice to any of the computers I've had.
I bought Drive Genius and Data Recovery from Prosoft to recover some data and then defrag everything across three internal drives. After Defragging my 80 GB boot drive, I go to restart the computer with it, since I had been running off of the Drive Genius disk. Nada, spinning wheel.
So, I boot off of the DG DVD again and run tests, and it comes up pretty much in the green, maybe a few problems, and it still won't boot. Since I have the disk's primary contents backed up elsewhere, I decide to swipe the disk and reinstall the system. Only problem is, I get the grey screen of death every single time I try to reinstall. I reset the motherboard and the PRAM, I even install a better processor, from a 1.733 GHz single processor to a dual 1.2 GHz processor, in case that was the problem. And I even tried the DVD drive that originally shipped with my computer.
I'm distressed to report that my might mac is still dead, though I'd like to think there's still a hope.
Any one have any ideas, or am I clinging on to a truly dead paperweight?

Answer 1

I didn't buy my computer at an Apple store, it was a refurbished system. I did, however, use a CD with the AHT image on it, and it said there was a default catch, and a mismatch, or something like that. Thing is that I put in a whole bunch of new stuff in it, and don't really think that the AHT could really do much.
I got a new G4 DA motherboard with a 466 MHz processor, just in case the mother board is actually out of whack. In any case, it should allow me to practically rebuild my computer.