G4s are great machines, especially the later models with nVidia graphics cards - the quicksilver, or better again late MDD models (anything post 2001 until they stopped making them in 2003). Forget about the early graphite ones, they're very slow. Forget about anything less than 733MHz & minimum 2gb ram too. (which is after getting really cheap, but double check if you are buying more ram that it will work in your machine) Dont bother booting them in classic os9 mode - stick with os x. Pick of the lot is a dual 1.42GHz MDD model from 2003 just before they brought out the G5. Early g5s are cheap too, but the g5 case is huge in comparison to the g4 & more often than not early g5s were low spec. Dont be tempted to open the case while the machine is running - fans cut out & the processor overheats. We are running a mix of 2002-03 g4's & 2007-08 g5's (& one early graphite iMac) at work, cant fault any of them. The g4's run nice & quiet & are bombproof. The dvd drives do tend to give up, but are cheap & easy to replace. Any replacement parts - psu, motherboard etc etc are eay & cheap to source off ebay.
The biggest downside with using apple stuff, is that occasionally you may have to visit an apple store, & have to communicate with apple store workers. They always have & always will be, without exception, a right pain in the hole to deal with, due to the fact that they are such clueless assholes.
