Greetings, congrats on your new gps, I was in the same position in December being a new owner of an x50v and dell gps. The dell software works great for street navigation, but doesn't allow you to add your own waypoints. Since then I've tried a few programs, and have since purchased a stand alone gps for caching (just don't want to damage my Axim). But I still use GPXSonar on my axim for keeping cache info on my ppc, and really like mapopolis for map software, just bought the map of my county for $10, software was free. One of the reasons I like mapopolis so much is the little utility that takes a gpx file and adds all of the caches to the map, so you can see caches as you drive around
I did try GPSTuner for a while, it could read in loc files and allow me to select one of those to navigate too (has a compass and distance). Worked pretty well. I've also heard some people that like:
http://www.visualgps.net/BeeLineGPS/default.htm
Good luck