G5 2.0 Ghz Dual will not boot

Hello all. I'm trying this as a last resort. I'm new to Mac, but a fairly proficient user/troubleshooter of Pcs. I've read hours of forrum posts on the subject of troubleshooting a grey screen/won't boot issue, but everything I've tried hasn't worked. I'm hoping you can help.
The problem is, this Power Mac G5 2.0 dual starts up, chimes, goes to a grey screen with the Apple logo and stops. No spinning wheel, nothing. I've tried Option + boot, it gives me the option to boot from different drives, including the OS install disk, biut when I select any of these, I get the same grey screen.
I have reinstalled the OS numerous times on different hard drives, swapping them out to see if that was the problem, but no change. I have the G5 connected to a different Power Mac G4 via firewire target mode. Using this I've repaired permissions, repaired disks, reformatted and re-partitioned 4 different hard drives, but same thing, it won't boot.
Currently the G5 is started in target mode, and I have the G4 booted from the G5's startup disk. The G4 is running fine from the G5's disk.
I also wanted to try the Apple Hardware Test, but unfortunately, I do not have the original disks that came with this Mac.
Any help and/or suggestions would be greatly appreciated.