Brand: since computers arent really made to run in a car, theres not really a brand, its rather custom made. ill explain.
i used a laptop (originally branded Aopen - regular laptop) then strip it down from everything - keyboard, mouse, cd-drive, monitor. basicly minimize the size to max, leaving only motherboard, bat and HD- as we all know the tundra is pretty packed together and there arent many empty hidden spaces.
then the operating system - i used XP Pro - which also has to be stripped down to the max as you need extremely quick boot and shut down times (5-10 secs).
Features: these are the features i currently have
80GB music/video player (any format)
however keep in mind that it is a computer and therefore features are limitless.
Cost: thats tricky because theres always the question of what kinds of part you already have and how good and creative you are with your garage tools, heres what i mean...
computer - my cost on it was $0 as i had a spare lappy. normally the range is $400-$2000 it depends how new and powerfull do you want it.
monitor - $300 - there are also motorized ones which are around $400-$600. (touchscreen)
GPS module - $50-$100 (had a spare one)
inverter - $70-$120 (i made mine)
start-shotdown module - normally $50 (i made mine).
volume knob - $60-$150 (made that too)
and alot of time, ALOT...
and attached are some pics, let me know if you would like to see more pics. and if you are interested in such project i will gladly help you (;