ok take a chilling break back.
Grab one of these: Foxconn D510 Nettop
that has more computing power than the space shuttle (mine is 4 cores, 64 bit - a wee overkill for the car right ?). It is the size of a VHS tape.
A DC-DC converter (P1900 or P2240 by Carnetix) will allow you to supply power to a wide range of PCs and smartly turn it on and turn it off. You may have to do some soldering such as to the power button to connect it to the smart power source ports. But in the case of the above unit that was easy.
Screen of your choice.
There will come a time when you will either forget to remove the tablet PC or you will feel either lazy or secure in your decision to leave it hanging there for 5 min while you run in the gas station or back to the house/office etc. In doing that you will provide an excellent opportunistic crime event. Not to mention that hanging that huge thing (the IPAD or tablet PCs) does take time; may reduce visibility and or inconvenience you in the safe operation of your vehicle. You only need that one time reaching in your glove compartment and getting your T-shirt caught in your hanging tablet to either ruin clothing or smash into someone. Additionally, in case of an accident, I'd would like to reduce the number of things flying in the car which are mounted in front of a driver (and that includes GPS units; radar detectors etc).
My advise is do it right the first time. Once you go on the CarPC route, you should have a budget but if you have 3-500 to spend on a tablet spend another 3 and get it done right for a number of reasons, some outlined above. And if you're gonna tell me that portability and being able to take it from one vehicle to another matters; then that's what a 5 inch Tom-Tom is for.
See the CarPC thread for details. The first 20 pages should give you the idea of how to mount your screen. Mind you, you can get a screen to already fit in there http://store.mp3car.com/In_Dash_Touchscreens_s/61.htm
or you may have to hack a little.
And no, a basic touchscreen interface akin to a smartphone is not gonna get you far for a CarPC. Recommend Centrafuse or RoadRunner (on the free side)