Have you considered..

[Anonymous]

belkin's nostromo 52 speedpad? http://catalog.belkin.com/IWCatProductPage.process?Product_Id=164714

to control?  another idea would be ebay hp's laptop remote control and use a media center ir receiver.

[Morely Dotes]

For the main system, take a look at the Maxterm Maxspeed thin clients; you can pick one up on eBay for less than $50 including shipping; model 8300B is a good choice. You can either replace the existing CF card with a larger one (16 gigs for about $35 last time I bought one), or mount a laptop drive on the inside of the lid (which I also did). The thing is already using an external AC-to-12VDC power brick, so you can just wire it to your car's power system.

For the controller, consider a numeric keypad. IT can easily mount on the center console or dashboard, looks elegant (compared to a gamepad, anyway), and can be operated without looking once you get used to the layout.

[admin]

Morely,

The thin client looks kind of neat, but I don't think I'll be using one in the car-computer project.  I'm going to need at least 2 serial ports, and I doubt the thin clients are going to support > 133gb hard drives.

I might get one anyway, though.  I can think of at least one other project where one of these would come in handy.

As for the controller, I know many like using numeric keypads, but I'd be swearing at mine.  Besides, I have no good place to mount one.  Playstation-wanna-be 10-button gamepads don't look very cool, but I find them really easy to operate.