It probably depends on the map, but I have a Garmin 76CSx, which is the marine version of the 60. I downloaded a map of the Palm Springs area from GPSfiledepot and once I figured out how to do it, it worked great. Just put it on a separate card so you don't erase anything you already have. They are only a few bucks apiece. Available at Radio Shack and other fine retailers.

GPSfiledepot is a great site. The forum members are very helpful if you have questions.