Congratulations on getting your Power Mac G4! I wouldn't try to measure voltages when the machine is on. The chance of short-circuiting something is too high, even for experienced electronics hobbyists. Just try it and see. Like you found, voltage levels can be found on the internet.
What I don't get is why people insist on keeping the files on the hd. I would just go ahead and format it and install a fresh copy of 9.2.2. You won't have to defrag it for a while, anyway, and when you do you have other Macs you can use until it finishes. For me, keeping existing software would bother me, since it isn't "mine". And you can always DL the software yourself.