ITX - Is nice for the size, but too slow for my tastes. If you insist on small size, this is the winner.
Shuttle is ok, but a waste of money for your application. You're only using the mb and probably can't sell the case b/c of the proprietary mb.
The ATX PC is a bargain. But again, too slow for my tastes. I use the carputer for entertainment including gaming. Next time your stuck in traffic you'll regret not being able to play quake III while waiting
I would go with an upgraded ATX PC (actually I did). Pick up a micro-atx mb with all the onboard goodies. I like intel for car pcs. They're not as sensitive to heat and can get away with low rpm quite fans. You may even be able to squeeze into the console. Depending on the chip/mb you pick you should be able to stay within your budget.