I did a crazy amount of reasearch into this matter, so I can help u out a lot. OK, so u need a computer that does not consume a lot of power. Basically what most people do is that they go with a mini-itx motherboard (EPIA -- The Via 10000, i think it is, motherboards are very popular). They have built in graphics and audio (but if u want good audio, i recommend using a Creative Soundblaster card). U will need a Mini-Itx case, as well. U need like 256-512Mb RAM, not more than that. Get all the other stuff that u need to (Like wireless card, GPS reciever, and all that, I cant remember right now). For ur touchscreen i recommend using, a Xenarc or Liliput, are very popular and Xenarc's are excellent from what i hear. U need a PSU (a power supply). U need a DC-DC power supply since cars are DC, so I am going to direct u to Opussolutions.com for that. What these power supply's do is that they are connected to the ignition wire and when u turn the car on the computer turns on and when u turn the car off the computer automatically turns off or hibernates or whatever (Very Very Cool and can save u a lot of time from using an invertor and hardwiring a power button into the cabin). I cant think of anything else from the top of my head. I pretty much covered all the important stuff. Also, one more thing, u need a program to run ur GPS, Music, Videos and all that good stuff. People use Roadrunner, or Frodo Player, Free.
I abandoned the project because I do not know how to do wiring in a car and knowing the unreliability of computers in general i thought i might have to a lot of troubleshooting. Basically it is a very very big time commitment but when it works, it is amazing.
If u need more information just PM me!! Hopefully it works out for u bro.
Also check out mp3car.com, they have a tremendous amount of information.