Well, I haven't found a set of parts yet that does exactly what I want, the stuff referenced above all comes close though. I'd like to get OBD-II sensor data (ala Magden or similar) as well as a good-quality GPS nav system, both using the same single touch-screen interface, mounted where the factory nav goes (or in my case, where the factory cubby is).

A quick trawl through mp3car shows that DashCommand seems to be the favorite app for magden like gauges in a pc, though there certainly are other options. I used centrafuse which had OBDII but it was more for diagnostic purposes, number and not virtual gauges.