a lot has changed over the years-- just in the last 2 years the carputer hobby has grown leaps and bounds over when i first became interested. mostly due to the higher consumer demand for tiny, and high powered computers.
the best place to start would be here: http://www.mp3car.com/vbulletin/advfaq.php
otherwise, just surf the store to catch up on the more recent tech.
the boards with atom integrated processors seem to be very popular for car computers-- mostly because most can be had for cheap, and do well in most systems.