Before you start to choose your carputer hardware, it is important to realise that there is a big difference between plugging your desktop PC into a mains supply socket on the wall, and powering your carputer from a car battery. With a mains supply, you do not need to limit (within reason), the amount of power your PC needs to function properly.
In a car, there are two limiting factors which must be considered when choosing carputer hardware. The battery and the alternator.
The way in which you intend to use your car PC will determine whether or not your battery is up to the job.
A car battery's principal task is to provide sufficient power to start the engine. If you intend to use the PC for long periods without the engine running, you may need to consider fitting an additional "deep discharge" (or "deep cycle" to use its correct term) battery. This will ensure the battery can provide enough power to turn over the engine for starting. (see the section on batteries for more details).
You also need to consider whether your car alternator generates enough power to keep your battery fully charged with the additional load your car computer represents. It is fairly obvious that the most suitable PC for car use is one which has a low power usage.
A good place to start choosing the hardware and peripherals to build your own car computer is the Motherboard and Processor section.
Use the index links below to get further information on hardware issues.