It's really not a very good idea to ask "what's the best system" for a few reasons.. but most importantly, everyone has a different setup. Most people think that their setup ist he best for them.
On that note... I have an epia mother board in a morex case the case fits neatly under my seat.
I use an opus power supply because it's a DC-DC supply which means I can hook it up directly to my battery, and it has advanced automatic on/off capabilities with your computer.
I have found that for mp3 playing, an IR reciever with a remote control provides a simple and cheap interface to change your songs etc.
I would recomend searching around the forums here, and especially looking at the projects section to get some ideas. If you ask specific questions you will recieve much better answers than asking such a broad question. This place tends to take the attitude that you should work hard to learn on your own and ask specific questions as you arrive at them. Most of your answers are in here somewhere if you look. Spend time reading here, it'll make your carputer building experience far more enjoyable, you'll learn a lot more, and spend a lot less