A few of us are using a board that works great for this...
I also have the M350 case with a M3-ATX power supply to power the following.
This board is made for harsh environments and has way more than you will ever need in your vehicle. WiFi, WIDI, Bluetooth, SATA 3, USB 3.0 and 2.0, Dual HDMI outputs at same time, etc etc... VERY nice board:
With this board I am using a i3-3225 processor which is a LGA1155, 55watt, 3.30ghz 3MB cache Intel chip with Intel Embedded graphics 4000. If you need more powerful graphics chances are you will need a bigger case with a larger power supply and graphics card. You will need a low profile cooling fan. I bought a 65watt low power fan for mine. I am considering liquid cooling but don't really need it.
I also have 2 8gig DDR3 memory cards in mine for a total of 16 gigs ram.
SSD 120 gig drive. You really want an SSD drive in a carpc because they are more tolerant to the temp differences and use much less power. Plus they are under $1 a gig now for 240 or smaller gig drives.
I also have a resistive 7" touch screen with this. If you want to use Windows 8 you will want to find a multi-touch capacitive screen which seems to be somewhat hard to find. If you do not get one you will not be able to use the gestures and it makes it VERY difficult to use with Windows 8 without a keyboard or mouse. This screen otherwise works like a mouse and works fine with Windows 7. If you can get your resistive screen to not have a bezel you MAY be able to use it with windows 8. My issue was not being able to access the charms which makes it almost impossible to use. It did come with a pen which might make it work but who wants to search for a pen when your in your car driving down the road to touch the side of the screen for charms. In my final installation I will likely use a couple capacitive screens and windows 8 but have to figure out the size screen for my truck yet.
In the end I get pretty good windows 7 performance numbers. I believe I get over 7's for everything but the Graphics which I get a 5.9 for both 2D and 3D graphics.
You will see this board in a few of the posters tag lines.
I have been told others have measured a similar setup running at under 50 watts at idle and not much higher at full load unless using graphic intensive programs.