Hello terry,
Panther
loves RAM. The more the better, however, it is also very picky about it's RAM. More RAM = faster performance = happier Mac.
Depending on the model, your iMac will take up to 1 GB of RAM. I don't know about RAM prices in Aus, nor do I know about the prices of used G4's and G5's, but RAM tends to be the least expensive option here in the winter part of the world (LOL).
Whichever way you decide to go, you have been given good advice by the other posters. As scapesuiter has pointed out, it depends on what you want to do with your machine.
A G4 will take advantage of Altivec enhanced software, which could mean having to upgrade your programs to make them faster. Your current programs can take advantage of a RAM increase without the added burden of software upgrades.
Hope This Helps
Jeff
Mini 1.25, 12 Al PB 1.5, PTP 250, WS PB G3, 3xiMac 400 (OS 9.1) Mac OS X (10.3.9) Wireless KB/Mouse, AEBS, 80 GB OWC FW HD
Message was edited by: Jeff Rushton (amount of RAM)