iTunes can convert your files to MP3. You don't need anything else. You just need to know how to set the conversion controls in iTunes.
In iTunes 6, go to iTunes:Preferences and click on the Advanced tab. Click on the Importing tab under that. Set it for the MP3 compression format you want. From the Import Using menu, select MP3.
From the Setting menu, choose Custom... and then 64kbps, 22KHz, mono. Click OK and OK again.
Then drag your AIFF file to iTunes. Control-click on the file and choose "Convert to MP3."
Viola, and MP3 file.
Once you have done this, however, you MUST go back to the iTunes preferences as above and set things back to AAC encoder at High Quality, 128kbps. Otherwise, when you rip new music to iTunes from your CDs, it will convert them into podcast quality, which sounds horrible for music.
Come to the GarageBand 2 discussion forum if you have more questions.