Although I am an Apple laptop user myself, I suggest that you ignore this advice about the iBook. Clearly you are in the market for a desktop machine. This is probably someone who owns an iBook and thinks that therefore everyone else should own one too.
At any rate, the Mac Mini and a Powermac G4 will both do you just fine for the sort of work you're describing. Mac OS X runs more than fine on a G4, especially if you have 512MB of memory in it (minimum suggested memory for OS X Tiger). Remember that the vast majority of Apples are still on a G4 chip, so it would be totally impractical for OS X to run anything less than perfectly on a G4 chip. This is especially true in the dual processor machines like the PowerMac.
If you're debating between the PowerMac and the Mac Mini, the info at lowendmac.com help you. It lists both the best retail deals for each machine (although as you know, almost no retail site can beat Ebay prices) and it compares the PowerMac and the Mini. here are two helpful and revealing articles about the PowerMac G4 versus Mac Mini question:
http://lowendmac.com/myturn/05/0511.html
http://www.lowendmac.com/ppc/deals.html
If it were me, I'd be tempted to go for a Mac Mini because they're all still eligible for AppleCare (they're less than 12 months old) and because most of them will probably come with Tiger pre-loaded. The PowerMac G4's weren't sold after 2003, so they're no longer eligible for a new AppleCare plan, which can only be purchased in the first 12 months of system life, and some of them might still be sold with OS X 10.2 Jaguar. By contrast, even the oldest Mac Minis were released in January 2005, so they're still eligible for warranty coverage and most of them probably come with Tiger installed.
