I am a very active musician and am and have been in several bands, so I have tried just about everything on my mac, as well as actual studio equipment. I have found that when recording with a mac, garageband '09 is definitely the way to go unless you want to spend a ton of money. The only setbacks I have found when using garageband to record are:
1. Limit on the number of tracks (With my Mac Mini, I believe this is 15...I think its a RAM issue, so you should get a macbook pro if you need a ton of tracks per song)
2. NO BOUNCING...this is a major problem, but it can be worked around. When recording with my mac (usually for experimentation and nothing serious) and need more than 15 tracks, I mix everything and then export what I have as a song, and then treat the exported song as a bounced track.
Hope this helps
Also, it seems obvious but you will need to get an adapter to convert the 1/4 to 1/8 (1/8 right? Brain freeze) input that will be the mic input on your MB. If you don't want to get fancy, these cost around $4 at radioshack.