Garmin has made things difficult by making their maps on the GPS kindof proprietary, however it is possible to upload custom maps from other programs. Personally, I use Maptech Terrain Navigator but I suppose that there area several other programs that will do the same thing possibly even for free. Just do a search for Opensource Topo Software or Opensource GIS Software and you'll come up with something. Now for getting the maps on the GPS, here are some instructions for that
http://vip.hyperusa.com/~dougs/GPSSM/. As I said, it can be done, it's just not quite plug and play unless you use the Garmin software. To be honest, I researched this before and after discovering how much is involved I decided that it wasn't all that important to have the maps on the GPS after all. I just print off a map if I need one.