Realistically I wouldn't buy from PowerMax. I thought about that a while ago too, but they really don't have the best deals. If you don't mind doing a little leg work ebay is the best option for used macs. I got a used 333mhz G3 for 50 bux, which can be upgraded to a G4 500mhz for about 150.
Like I said, it will take some leg work to find that killer deal on ebay, but a few hours spent to save a few hundred bux is well worth it.
You may even want to check the local university for a used G4 right now is about when they sell old computers to students/the public for deep discounts, you may find a used G4 with Jag on it (Apple gave it away to universities for free) for a 2-300 bux.