Aymat, I've mentioned it before, however not in this thread, the Traxaudio Mstation! I managed to get it installed in my car just intime for Gaydon. Anyway, now have it fine tuned and pretty near perfect for my needs.
It is essentially an IDE hard drive, standard is 10 GB, in an enclosure which can be mounted in the boot. It is then controlled by a small wired reomote, which displays song title, artist, time etc. It'll hold thousands of songs, nice to control, as you arrange the songs however you want, in directories on the HD. I've mainly got them by artist and album. You can make playlists from your songs too.
The thing I like is that you can keep your headunit, as I really like my plain standard looking Panasonic headunit, but wanted mp3. It is connected via an FM modulator, as unfortunately I didn't have any RCA input sockets on the back of the headunit.
I still have the single CD option in the headunit, the 6 disc changer in the boot, and also the MP3, so loads of choice. The MP3 is great as there's no messing about changing CD's or carrying them with you. All you do is take the unit out of the carbay and connect it to your PC via the IDE bay and update like an extra hard drive, drag and drop files.
It's currently mounted in the centre console storage area, I made a small mounting for it, so that I still have storage behind it, but I'm not 100% happy with it, as in 5th gear the display is slightly obscured, so when I get round to it I'll move it up to where the extra power sockets are and make a new casing to mount it in.

Panasonic Head Unit.

The kit, mid bottom, is the unit which can be mounted in the boot, glovebox, under the seat etc.
Anyway, there's another option when it comes to MP3 ICE.
Pete