I just purchased a used Mac mini that has a 1.42GHz G4 processor, 512MB of RAM and an 80GB hard drive. It's pretty good for casual web surfing, emailing and chatting and even manages to play a slightly hiccuppy game of Warcraft III (though be kind and don't even bother installing WoW on it).
I got the Mac mini, an iBook and a MacBook tested using the latest Xbench (1.3) and posted the results:
iBook - 933MHz G4 / 640MB / 40GB
MacBook - 1.83GHz Intel Core Duo / 1GB / 60GB
Mac mini - 1.42GHz G4 / 512MB / 80GB
Yeah, the competition wasn't really fair but the iBook was really pathetic I must say.
Surprisingly, this 1.42GHz didn't come with Bluetooth or AirPort, so that's going to be another $100 and 1GB RAM is going to be an additional $100-something.