Honestly, I would upgrade. Thinking about it now, I don't think that thin client even meets the requirements for DVD playback without dropping frames. And I imagine navigation software being pretty hard on that little machine too.
I used to run a low power PC in the car, but now you can easily go dual core and keep the power requirements reasonable. My current CarPC is AMD dual core at 3GHz, 4gigs RAM, and SSD, running Win7 32bit. You don't really need 64 bit in the car. I'd stay with 32bit, just to avoid any potential software/driver issues. But definitely SSD all the way. Nothing benefits an entire PC's performance like an SSD. My setup is just a 64gig SSD, and it's more than enough to install Windows7, apps, and 10,000 songs. And I've got a 32gig thumb drive if I need more music space.
In WMP, if you got sound but no video, make sure you turned the visualization back on. I believe WMP treats any video playback as "visualization". But really, do we need DVDs playing in our cars? (i know, i know, for the family, or kids, or friends, or just for the hell of it.)
A lightweight front-end leaves more RAM and CPU power available for navigation apps, or other crazy stuff we do in our cars.
OBD through DriveLine won't use up much CPU at all. The serial devices we use (GPS, OBD, radios) don't require much processing. It's mostly just monitoring the data coming in and occasionally sending some data to the unit.
If you go with the Mini-ITX, just remember that there is no such thing as "too many USB ports". I think USB hubs are garbage. Plug as many things directly into the motherboard as possible. Also, a motherboard with newer/better integrated graphics will be able to offload some video-processing onto the graphics chip.