I'm running a carpc in my 32 using a VoomPC-II enclosure, it's a 1.2Ghz onboard mini-ITX with 1Gb RAM, 120Gb 2.5" HDD and a slim DVD drive. PC cost about $500 to build and the touchscreen was more than the cost of the PC itself.
I have a trackball stashed to the left of the centre console for when I need to do something that is a bit hard with the touchscreen.
Don't have a keyboard in the car as there is no need, have tried a mates 3G broadband dongle and it's mint. PC is about the size of my amp and resides in the passenger footwell up against the side of the centre console, gives me easy access to DVD drive and USB ports.
It's a fairly removable setup. You could get pretty fancy if you wanted to graft things like the screen, USB ports, slot-loading DVD drive into the dash.
I use it for music (outputs to the aux port in my deck) and for diagnostics (remapped stock ecu, have consult cable permanently plugged in).
For the screen, make sure you get one with VGA input or it will look like shiet
Edit: have been contemplating organising a group buy on the VoomPC setups (shipping is a killer) and I have a local supplier of 7" non-motorised in-dash VGA screens (they have others too).. not sure if there's enough interest though.