*gtrdave offers PianoWizard a Mac-brutha fist bump*

I'm listening to O Holy Night right now...really, really nice! My sub-woofer is happy

For making mp3s I normally use iTunes.
You can export from GarageBand right to iTunes as an uncompressed AIF file and then convert to mp3 (or AAC or Lossless and maybe another format) while in iTunes.
For decent quality you may want to set the rate at 160kps or even 192kbps rather than the standard 128kbps.
To change the settings, go to iTunes - Preferences - General - When you insert a CD: - Import Settings...
Another option is Audacity. It's freeware and, like GarageBand, is multitrack recording and playback capable. The available mp3 encoder (also free) is good if you need to really compress a file down to nothing. I use it for our pastor's sermons; we record them on a MacBook in GarageBand and then export to iTunes as AIF. Then I open it in Audacity and convert it to mp3 at 16kbps. It squishes the 50 minute long file down to about 6meg in size which is suitable for voice streaming and download without losing a ton of quality.