DeLorme's Topo USA certainly is one choice (I have version 3.0), but the Garmin MapSource software that came with my Street Pilot 2730 GPS is excellent, and the routes (as well as waypoints) can be loaded from the software to the GPS (or vice-versa). Both MapSource and its road database (same as on the GPS) are updateable from the web (the database for a charge, of course). DeLorme's Street Atlas is good, too, and while it doesn't have the topographic info, it has a bit better (at least in my versions) database of points of interest, motels, restaurants, and other businesses, along with parks, etc., than does Topo.
The 2730 has, many times, helped me find motels, gas stations, restaurants, grocery stores, etc., especially helpful in places like Vermont where road signs seem to be unknown for businesses, but still quite useful elsewhere.
So typically, for a trip, I'll plan the route on MapSource then, once the trip is finalized, dump it into the 2730 (USB connection) for use on the road. I can do this from either my laptop or my desktop machine, so changes on the road are perfectly feasible (of course you can change it within the GPS, too).
So MapSource seems to me the most useful (for me), but I use Topo USA (it's also on my laptop) when I need topographic information and, if I choose, I can hook the GPS to the laptop and let Topo track me, as well as the 2730 itself. I can even use Garmin's nRoute (free download, but mostly usable only with Garmin GPSs) to track my route on the laptop (I don't do that very often).