All 3 of your questions are very basic. This tells me that you haven't done any searching. I've provided you with links that will get you started, but I recomend you start here first:
http://www.mp3car.com/vbulletin/newb...ere-start.html
and
http://www.mp3car.com/vbulletin/faq-...-faq-faqs.html
1)
http://www.mp3car.com/vbulletin/faq-...tereo-amp.html
2) You need to search. Look at the screens that are here on the store. You need to determine what you want: to mold a screen in for a factory look or to have a fold out screen.
3)
http://www.mp3car.com/vbulletin/power-supplies/
Your idea to use an Athlon 64 isn't such a good one. It generates too much heat and needs too much power.
Again, you need to do some searching to figure out more detail as to why this is the case and why I'm saying this.
We're here to help you in your project, but we do not spoon feed. There's a very legitimate reason for this:
http://www.mp3car.com/vbulletin/faq-...n-feeding.html