iTunes will allow you to store your music in mp3 format:
1. Launch iTunes.
2. Edit -> Preferences -> Importing -> Import Using -> select MP3 Encoder, then click Ok.
From this point onward, whenever you rip tracks from your sources, all of your tracks will be stored as mp3; however, this doesn't apply to any tracks that you purchase from the iTunes store I believe; you have no choice, AAC is the only game in town here.
For your existing collection of tracks already ripped to iTunes, I'm thinking you may need some sort of converter, something like this:
dBpower AMP.
What I would do is go over to
iPodLounge; Scan through the FAQ and then maybe post a similar question on the forums