You can use that old eMac for lots of things. I personally use my Power Mac G4 for GarageBand and Internet use. I also have a few games as well, like Shrek 2 (It sucks compared to the PS2 version though), SimCity 4 (Doesn't perform the greatest) and Minecraft, which runs with a certain beta version of Java.
My Power Mac is also one of 4 computers I have with my entire music library. I keep it hooked up to my surroundsound system so I can blast music all day long.
Internet also works pretty well too. YouTube is alright, just use Camino instead of Safari because Camino can disable Flash and you can click flash objects and they'll play, so it's faster plus it can disable Internet advertising. You also can't watch YouTube in anything higher than 360p, I find.
My Power Mac is a dual 1.25GHz with 2GB of RAM and OS X 10.4.11.