I got months of enjoyable reading from this website before I decided to build my own. Regretable my entire project is on hold due to financial contraints. However, you should read up before making any decisions.
Alot of people build a mini-pc using Via Epia motherboard which has a built in low-power, low-heat CPU that is about equal to a 700 or 800 MHz computer. These motherboards are about 180 mm x 180 mm or smaller. Used with laptop hard drives and the such, many users make complete mini-desktops that run off the car's 12v battery through a power regulator and a start up controller to the computer.
A car's battery voltage fluctuates from 7 to 14 voltes so a regulator is needed to feed the computer a steady 12v. A start up controler will start the computer and turn it off based on your car's ignition status.
Some choose to run a more traditional system comeplete with a 120v ATX power supply. This is wired to an inverter to convert 12v DC of a car to 120v AV of a home outlet then the computers powersupply takes the 120v AC and converts it back down to 12v DC for the computer's need. Some report problems with inverters. They are inefficient and known as the cheap alternative. Though if you are able to set up a system correctly, you could end up with a much more powerful system. S startup/Shutdown controller for this type of system is much more difficult and will probabaly need to be made from scratch. It's not impossible or particularly hard, but would require a bit of work.
It would probabaly be easier to use a conventional startup controller wired to the ignition lead (which is 12v when the car is on and 0v when the car is off. It should be used only to trip relays and not to power anything). Then install a remote car starter. I know it is expensive, however you get increased range and if you start your car while walking toward it, by the time you are in it, buckled and situated, the computer will be started, or close to it.
7" 16:9 VGA touchscreens are common. Xenarc touchscreens gor for about $400 and have a reputation for being excelend in quality. Lilliput touchscreens are the cheaper common alternative and go for about $300. Many report going through several Lilliputs trying to save money and end up spending much more. Other's report near perfect Lilliputs (known as Lilly's).
These screens run on 12v and are often powered by the same regulator used for the computer. The touchscreen is USB.
The following are common accessories used in addation to playing MP3s and videos:
GPS- small USB GPS units often are used for navigation
XM- XMdirect can be controlled via serial cable
FM- Radio control found on some TV tuner cards, other solutions in the making.
TV- PCI tuner cards
Wi-Fi- Wireless cards used to wardrive, remotely connect to the internet, and wirelessly upload new data to your carputer from your home
Internet- Laptop cards sold my cell phone carries, Sprint, Verizon, expensive and slow.
OBD-II- This is a means to interface with your car's onboard computer getting vital information from it, such as speed, techometer, gas, milage, warning light information, etc.
Most interfaces are made to work with the touchscreen needing no keyboard as many systems dont have a keyboard. These interfaces are known as Front Ends. Choosing one is a matter of personal preference. Frodoplayer is very common and currently it is constantly being worked on by Frodo, who is sort of a god in this forum. Mobile Media Center was my favorite for a long time. There are sub-forums here strictly for front ends. There are very limated front ends for linux and the mac. I would imagine however, with the imergance of the mini-mac that there will be some mac development. From what I read the mini-mac seems pretty well suited for a car PC.