I have a 20GIG one, and I have about 1600+ songs and still have plenty of room.

http://www.enfig.com/ makes a bunch of cables/adapters that can get your iPod to connect (with or without power) to your cars stock radio, and allows you to control the iPod from your head units controls.

You can also use a tape-adapter, or you can use an auxilary input (headphone jack) if your head unit has one. You don't need an "mp3 head unit," as everyone else stated.

Also, while the iPod is mainly for MP3's, in the end it's nothing more than a portable hard-drive and you can put any filetype you want on it, and transfer it from computer to computer by connecting to the computer via USB or Firewire, whichever you choose.

In fact, if you get the Windows-formatted iPod it will even work with your Playstation2 as a "USB Storage Device."

It is recommended (for ease of use) that you use iTunes to transfer all of your MP3's over to the iPod, because it's basically as simple as adding your MP3's to a playlist of your choosing, and telling it to transfer that entire playlist over to the iPod, and to keep the iPod updated with that playlist at all times. However, it is not necessary -- you can transfer the files manually if you wish.

And, any MP3's will work -- not just "iTunes store" MP3's.