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.
Please contact me if you would like to use this in a project. We can discuss the details.