> It's got a 1ghz G5 PowerPC chip so I have to run Mac OS. I can honestly say I
> have no idea what I'm doing. Anyone here good on Macs?
You're going to max out at OSX 10.5 with that one - 10.6 on up has dropped PPC support and Apple's basically giving the non-Intel stuff out there the heave-ho. Either way, I'd recommend maxing out the RAM (1GB) if it hasn't been done already. OSX 10.3 is dead, 10.4 is on its way out, and 10.5 is your best bet for it.
Also, check http://www.apple-history.com - any hardware details you need are in there (look under Family, iMac, Early 2003).
What exactly do you want to do with it?