Here's my $0.02, hope it can help you out.
I've recently acquired an iMac G4 (17", 1GHz, 512MB RAM, 80 GB HDD, 10.4.11). I use it as my secondary computer, since my desktop PC is on its last legs. For what I've used it for, the machine does alright. I've surfed facebook, browsed forums/ web, listen to music, iChat, wireless printing, etc... I'm sure adding additional RAM would help but its usable. Youtube isn't the best, but it works. The fan in the top of unit isn't too loud, but its noticeable if you don't have something else going on to drown out the noise (doesn't bother me though). It still seems like a pretty capable machine, but then again, I haven't tried to do anything CPU-intensive on it. I plan on utilizing it more after I get an external HDD and more RAM.