Hate to be a party pooper, but I'm going to have to say no.
My Lombard PowerBook is a 333MHz (its practically the same spec as the original iMac) but it has 512MB RAM in it. I have struggled to get it to run iWeb. It still won't run usably.
OS X is RAM hungry. Even if you stick to Panther and an early version of Garageband, your'e going to struggle. Panther alone will be slow with only 298MB, Tiger even worse. And music apps are RAM hungry too. Even if you manage to trick the installer into installing it in the first place, I doubt it will load fully/properly. If it does, it will beachball for days just opening projects.

Minimum requirements for GarageBand 1 as I recall are a 700MHz G4. I wouldn't waste too much time trying this.
Better off with a classic OS and an old copy of Cubase or Protools.