If you really want to fire up GarageBand, I think the Stealth Plug from IK is supposed to be pretty cool. To do some starter recording, I was going to check it out. I can afford to lose $100 on trying it and it comes with a pretty good load of enclosed software like Amplitube (which I read was compatible with GarageBand) and it's all plug-n-play with a Mac.

Just a thought.