Sorry, your questions are WAY too broad - please be more specific as to what you want to accomplish. The car pc system can be split up into 3 main blocks (without any additional bells and whistles):
Computer block - includes all your computer hardware and an automotive power supply. Most people have it stowed away in a trunk, under a seat or in a glove box. Some manage to squeeze it right behind the double din opening. As far as brands go, mp3car.com and mo-co-so.com sell prebuilt systems that include case, power supply, mobo, memory, hard drive etc.
Monitor - this usually goes where your radio used to be. Hopefully your car has a double din opening. Most people buy Lilliput brand LCD screens - they range between $130 for a basic vga model to $700 and up for transreflective units. You can buy them pre-fitted into a double-din-friendly bracket so all you have to do is get a double din bezel for your car and put it right in
Amplifier - if your radio was in the double din space, you clearly won't be able to use it anymore, since it will be replaced by the screen. You can either relocate the radio (if your dash has another single DIN space) and use the AUX In to feed the audio from your computer to the radio's amp, or buy a separate amplifier. I can't really recommend any brands, because this part isn't carputer-related
There's also a Black Box Enclosure - basically a project box that can accomodate a lilliput monitor, a nano-itx or pico-itx motherboard, a hard drive, a power supply and even a small amp - all in one neat package. Ideally, what you end up with is something that looks like a brand name navigation unit, only instead of crappy Windows CE, you can run Windows or Linux. That's what I'm attempting to do and it's pretty tough - I suggest you do a lot of research before you decide to go the All-In-One way.
Hope that helps!