Boostman I dont see the point in adding a computer to the D3. First of all, The best input you have on the D3 RCA Cable, Which sucks. The D3 does everything you need accept internet. You need to think about what type of system you want and start from the ground up.
For me i was never happy with even the best Major touchscreen car radio makers such as pioneer. I think using a mac mini and a touchscreen vga is the best way to go. nothing looks as good as frontrow and if you want a hdtv tuner go with the eyetv 250 and use the mac remote.