monty,
This thread might help you out a bit.
I personally don't think there's anything you could do on a mac that you couldn't do on a PC for around half the price, but having said that, I have never looked at Garage Band and it might be an incredible program. Rockerbob and Neilsonite are two of our members that I know use macs, maybe one of them might want to pipe up with some advice... but I would go and read the thread I mentioned above first.
Cheers
Clancy
"I like work: it fascinates me. I can sit and look at it for hours". Jerome K. Jerome