My current system is based on a Shuttle XS35V2 system. It runs an Intel Atom D525 1.8GHZ dual core cpu and 4 GB DDR3 ram. It is currently installed in a 2003 Mitsubishi Montero Sport SUV. It is normally used as a NetTop system or HTPC. I've done a few modifications such as adding a 3.5mm mini jack tied into the power switch and rewired the wireless network card to switch it from the internal antenna to a rear panel mounted SMA pigtail and rooftop external antenna. Power supply wise, it is essentially a laptop type system with an external 19vdc brick. I currently power it from a tiny little 150 watt dc/dc variable output boost converter adjusted to a 19vdc output wired directly to the car battery. The Shuttle is mounted using their vesa mount bracket on a rear side cargo storage door in the back of the Montero. It locks into the bracket with a single bottom mount thumb screw. I really love this little PC since it sips power, has full passive cooling (ie. no fans so no dust to get sucked into the case).
What is different about my overall approach is that I am using a much more distributed approach to the car PC. Instead of using a single combination power supply/startup-shutdown controller, I use several individual small dc/dc converters and an arduino. One major advantage to this is cost. The 150 watt dc/dc I use to power the main pc was only $9 on EBAY. I have had no issues during engine starting with it. My arduino, front USB hub, front relay module and rear relay modules are each on their own tiny little dc/dc variable output converters that were only $2 each.
The arduino handles my pc start up and shutdown, as well as monitoring battery voltage to completely shut down the system in the event of battery drain. For start up and shutdown, the arduino monitors acc power as well as 5v from the pc usb ports to know when it is up and active. When I turn on the ignition, it sends a 5v signal to the rear relay module to pulse the PC power button. The arduino and pc communicate over the USB connection. A small program written in VB allows me to select which option and how long after the car is shut down to apply it. I have a overhead console mounted LCD display connected to the arduino by I2C bus to display the day, date, time, inside temp, outside temp and any other info I choose to implement. I also have an I2C bus real time clock connected to the arduino that is checked against the clock in the PC for syncing them. The arduino can monitor the car and perform basic functions even if the pc is in full shutdown. When the PC is sleeping, hibernating or off, the arduino can shut down all front end USB gear.
All of my audio is handled by a front mounted external USB audio card driving a tiny little 45 watt x 4 class D Alpine amplifier that fits in the dash behind the LCD touch screen. I have an HP HD-4110 webcam attached to the mirror mount arm that I use as both a dash cam and noise cancelling microphone. I use a Visteon HD Zoom radio tuner module with a MJS Gadgets USB interface cable. Other components are a TomTom Bluetooth GPS reciever that can actually pull a good signal mounted inside the cab in the overhead console. It has a rechargable battery built in so it is entirely portable but I power it off yet another little dc/dc converter. OBDII input comes from a generic ELM327 Bluetooth module.
Some of the projects I am working on are further implementation of the arduino for sensing and control functions and sharing data between it, the pc and vice-versa. My current project is connecting a WII nunchuck accelerometer board to the arduino to supply physical orientation info to the PC to display on the LCD touchscreen as a 4x4 inclinometer plugin for Centrafuse. A future project will be networking a couple of Raspberry PI boards to drive rear touch screen displays. One idea I am toying with is using a handful of networked Raspberry PI boards as a distributed Car PC system to replace my Shuttle. One would handle display and control, another would handle audio, another video, another navigation and so on. This type of arrangement would be very flexible, modular and above all, inexpensive since the PI's start at only $25 each. By using multiple networked boards, they could be powered and booted individually as needed. They could all boot and power when the car is on yet keep one running all the time to monitor the car and stay connected to the wireless network. Imagine the possibilities.