I don't know much about Garage Band, but I do know about the free, open-source, mult-track recording software, Audacity. It's found at: http://audacity.sourceforge.net/
I've used it to record my own tortured-artist music and it's pretty simple and straightforward. I record directly onto my laptop with no fancy hardware. It's definitely DIY.
Depending your podcasting vision, you might already be good to go. You might consider background or intro/outro music. An electronic keyboard with a range of sounds and percussive options would be a good cheap way to go. You can probably pick up a low-cost Yamaha for ~$150 brand new. You never know what you'll find on craigslist.org or other used sites.

