Check out: http://www.fit-pc.com/web/fit-pc2/
I'm basing my current Car PC around it.
- Low power: 8 watt full load
- 2.5inch sata slot for SSD/HDD (You slide the drive in, screw it in, close it up. Seals real tight, drive can't move
- Made out of metal. I can't identify metals well, but I bet you could chuck the thing at a wall if you wanted to
- For ruggedness: the power barrel connector has a mounting screw on its side to ensure it doesn't fall out
- For auto-on: Ability to set it to turn on when power is applied, so you should never have to open it
- You'll want to feed it a regulated 12 volts, so you'll need a power supply.
Now that I'm using this thing, I would say if you're fine with Atom, it's the best package, but that's an opinion, so of course, let others weigh in.
Note that it does only have HDMI/DVI output and its HDMI->VGA adapter accessory only works with screens that provide their DDC/EDID information (on windows) so it will show your bios screen loading, the boot screen, and then dissappear at the login screen unless no graphics drivers are installed