I did some research into PocketPC mapping software. My requirements might be different than yours, as I would need Canadian maps for instance, but the following looked really good, had great reviews and great features:

Destinator: http://www.destinator1.com/
Routis: http://www.deluoelectronics.com/customer/product.php?productid=2&cat=15&page=1
TeleType GPS: http://www.teletype.com/pages/gps.html

For car use I'd definitely use a fixed antenna mounted in/on the car. And/or a GPS unit that's permanently wired in so you don't have to fiddle with any wires for power, etc.

Bruno