Congratulations . . . and for the record while I am an unabashed Magellan lover just you really can't go wrong with buying a Garmin.
As mentioned Mapsource is a more detailed map for your GPSr . . . while more detailed maps are never necessary to go geocaching, I often find that the more detailed maps are extremely useful . . . especially if you have mapping software that offers turn-by-turn directions to get you close to the cache while driving . . . or conversely I use my more detailed topo map while ATVing and sledding to see where I am. Trust me . . . not needed, but once you have it you'll never want to use the base map packaged witht the GPS ever again.
"Courage is not the absence of fear, but the realization that there is something more important than fear."
"Death is only one of many ways to die."