I use a 5yr old Garmin III, to navigate with. I run it out of the cigarette lighter, have it mounted on the top of the dashboard, near the left side of my steering wheel (no external antenna). It works great and has most of the roads that I am going to be on already included on the self contained map. The amazing thing is the GPS indicated speed that is calculated from the 12 satelites is often closer to my actual speed, when checked by radar, than my my truck speedometer.
I also use the Delorme programs Map N Go, and Street Atlas to plan trips to plan routes. I use Delorme Topo USA to look at the routes. This program has a pretty good Topographic map of the complete USA on one DVD. You can put the program into 3D perspective mode and adjust the angle of view to about 15 degrees and scan the proposed route and it looks like you are flying over the landscape at about 2000'.
RoadKing Moe is correct the Street Atlas does take up a lot of room. If you don't need every street in the cities the Map N Go program would probably be a better choice. It will also hook to a GPS reciever and allow you to keep track of things on your laptop screen.
However I have never hooked my Garmin GPS to my lap top mostly because the built in maps are generally adequate, and I didn't want the clutter of the laptop.
I detach the GPS and use it when we hike, I generally pre-load several waypoints on the trails that we want to hike, as well as mountain tops that I believe will be visible along the route. We turn on the GPS and catch a reading anytime that we stop for a snack ( approx. once an hour).