I built a carpc based on Travla 134 housing (size of radio) and Via Epia M10000 Motherboard. It has USB and DVD reader and Hitachi ultra HD of 30 Gb. Screen is an 8 inch touchscreen but has only 640 on 480 physical resolution so screen is not very clear. If you take a TFT screen go for at least 800 x 600 physical resolution.
GPS is a usb Holux GR213 Sirf-III that works fine.
Software is Ozi and Autoroute 2003. For Ozi I have Holland, France and Portugal in 1:25.000 and Belgium in 1:50.000. Still looking for UK etc
Still could not figure out where to put it: under the cubby box, in the cubbybox or against the roof
Last week I bought a tabletpc type Fujitsu Stylistic LT P-600 running XP. It works perfect except the screen is very difficult to read so pretty useless in the car. Now looking for another screen to put in.
Tablet pc is a lot easier to install and to take out the car, has build in battery so you also avoid problems of not enough current when starting engine etc