After lurking in here for a while gaining lots of ideas for my own install I thought it was now time to share the results with you.
Specs
AMD64 3GHz
MSI K8MM3 motherboard
512GB Ram
40GB Hard Drive
7" touchscreen
First off here is my car, a 1998 318i
My original dash
I wanted as small a case as possible but couldn't find one small or slim enough. I ended up installing the PC in an A4 box file. I mounted this on a wooden stand and fitted it into the recess in the boot where the first aid box used to live. I'm really pleased with the result as it is out of the way and doesn't interfere with the load space in the boot. I'll remove the case and post some pics of the inside later.
My screen purchased from Ebay. I don't know the make but I'm really pleased with it. It came with a headrest mounting and this turned out to be a perfect tight fit in the dash. Great news as I didn't have to do any cutting or fabbing a mount etc. The screen is a really tight fit but I can take it out in seconds if I need to.

As the PC is running the dreaded Windows it was a necessity to fit a reset switch in the console. I also fitted a power switch in the 12v feed to the M2-atx so that I can choose to have the CarPC switched off if I want to.
I ran all the cables up under the centre console and terminated them behind the screen. I've fitted a 4 port USB hub for the touchscreen, bluetooth and wireless keyboard adaptors.

You'll notice that I have fitted my sunglasses case with velcro to the side of the centre console. We get that much sun in the North of Scotland

that I just couldn't do without my sunglasses being handy

I fitted another USB hub with velcro under the armrest. Ideal for my flash drive when transferring music onto the PC
I hid my GPS receiver in the access panel for the rear brake light on the rear parcel shelf.
I've been running the system for about three weeks with no problems. The only thing is that I now want to move the screen to the top as Enforcer has done. This will involve a lot of modding and relocating the head unit so it will have to wait a while.