Depending on if you are near any major cities, you should try out craigslist.com. You can find them cheaper too.
But yeah PPC is going to be dead and $500 could be spent on a 2ghz C2D mini and that would run circles around any G4, even some G5's
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Only good thing about G4's is that you can run Classic if you need to run any of the older programs. The FW800 MDD will not boot into OS9 though. The dual 1.25 MDD without the FW800 port were the last of the G4s to run OS9.