FORGET OS X on that machine.
It would be an experiment in willpower, nothing more, just to prove that it can be done. It may even be able to do something useful, but audio will DEFINITELY NOT be among those things.
GarageBand's minimal requirements, btw, are a 600MHz G3 - and that assumes the appropriate surroundings - motherboard, system bus, RAM, etc, etc, etc., and only covers a few basic tracks and *maybe* a software instrument or two.
You won't even be able to hook up the tascam without an extra USB card.
Stick with OS 9, add OMS for MIDI (free), and find an old, old version of Logic or Protools Free or something, if it absolutely, positively HAS TO be that machine. Good luck finding a Mac serial MIDI interface and a functional audio card.
Advice:
Keep the monitor, and get a Mac mini. Upgrade the RAM as budget allows, but realize that RAM is necessary for audio.