I reccommend the via series because ultra low power, and you can get fanless versions too. My fanned version is silent enough though.
By media centre do you mean XP MCE? Wont integrate GPS very well. In a touchscreen car PC, those little check boxes and such are hard to hit with a finger.
I use RoadRunner, but there are many other frontends. I like RR because it is easy to setup, has the most support (skins to customize, plugins, ect), and it is free. Or you can shell out $200 for a software called StreetDeck, but I presonally think it is for those to lazy to install a free program.