Score!
Here's the info on that 2003 model:
http://www.apple-history.com/?page=gallery&model=g4_800&sort=date&performa=off&order=ASC
I'm using a dual 1 GB one that I got for free, using Cubase 4.
A couple of things you might not know about:
If your video card is like mine, it's got the special connector for the Mac monitor and also a standard monitor connector. You can use them at the same time. I have not found 1 monitor that won't work with a Mac. So find some POS freebie monitor, it could be a HP or Dell or who cares, and you can hook up the second monitor. It can duplicate your main screen, good for a monitor in the second room, or use it as an expansion - good for edit modes... great if you run Photoshop. I have 3 screens hooked up to mine - a third by my drumset (with it's own keyboard and mouse).
Also, any POS hard drive seems to work with a Mac, even ones from PC's. So find some garbage computer and put in a second hard drive if you don't have one.
I put a $30 (must be cheaper now) Belkin USB card in mine that gave me 5 more USB ports.
There's a free app called MidiPipe which is a very powerful tool that seems to work with any Mac app.
Yesterday, for instance, I programmed MidiPipe to do this: Every time I select a voice on my synth, it sends out the reverb, chorus, delay and transpose settings (correct 8va) for that voice. I've wanted that for years. A great app that I don't think they have a similar one in PC. It can use a prg change or cc change to send out any Sys Ex message (!).
You can program Midipipe that everytime you hit a certain cymbal pad on your electronic drumkit it would send out an email to a friend, start your car or phone the police with a predetermined message, useful stuff like that. I don't know any other app that can do that or has that potential.
There's also a freebie (actually $15) called QMidi which is the coolest MIDI playback program and is Mac only. I've been using it for gigs for years. You can program it so that it will play the song and display the sheet music and turn the pages at the right time. I have it programmed so with every song (instant access) it displays the lyrics and chords. It works incredibly. I would have to say that so far QMidi is the best music app I've seen on a computer and by and large no one has ever heard of it. It's deceptively simple.