I picked up the small Garmin ETREX: it only weighs around 6oz, and connect it to my notebook and watch my own inflite moving map... It works really well, but you can not connect an external antenna.
I use microsoft streets and trips, but the DeLorme Road Warrior with voice seems interesting to try for use in the car.
I tried the Sony Skymap for use in the car, but it does not correct itself if you make a wrong turn: it has since been "e-bayed".