I've been watching Katie over her shoulder. Not a fan of garage band but it's fun to watch the learning process in action. I should show you some drum sampling tricks next time I'm over. You can chop a sample from youtube using a browser based application, trim the sample further using audacity, then layer the new sounds in garage band. When you have a few sounds you like in garageband you can then export those sounds, resample them and keep layering sounds until you have all new sounds from old ones. Layering 101. It's a great way to get nice full drums.

I also recomend you invest $300 in a pc and just build a decent desktop to do work on. My cheapo can run circles around your Macbook. Back up you're data and I'll show you how to transfer it from mac to pc.