Answer to: Will not load operating system...install another harddisk and put a fresh osx install on it. Then migrate your data. that if it's the harddisk.
Answer to: Grey Screen at start up. No Single User,Had the same problem with my 2.3 dp and my problem was the memory. pulled ot one pair and voila. i'm working on it right now. so try running on minimal memory and go from there. suk6 did it a while, and failed again. reseated processors and cleaned connectors (with sanding paper :+) ) up and running again hope this has no follow up and my g5 it stays up. .