You should try
www.macofalltrades.com
they really have some good prices on cheap macs.
I don't know if you'll find a G4 there in your price range, but you might
find a bit faster/better imac for around $200. If you could save up the extra
money, I would try that. Good Luck! And stay away from ebay, I have heard
of a few people getting scammed there recently.