Most of this has been covered but in case it helps.

There are 5 kinds of "data" in your Garmin 60: maps, POI (points of interest),  waypoints, routes, and tracks.  Maps determine the picture you see on the screen.  POIs are those points that the maker of the maps has included as points of interest (duh).  Waypoints are points that the user determines are of interest.  Routes are an ordered collection of waypoints, again created by the user.  A track is a sequence of points indicating where you have been, generated autmatically by the GPS (you can create tracks in software, but that is more advanced than needed for this discussion).

A garmin 60 comes with a basemap, but you can buy more sophisticated maps like Topo canada and topo usa.  Mapsource is a program for moving maps, waypoints, routes, and tracks between your computer and your GPS in either direction.  I have an older garmin 60 but it appears to work as other have reported about the newer ones.  The basemap is never erased.  Each download of supplemental maps erases any supplemental maps already loaded on the GPS.  Moving waypoints, routes, and traacks to the GPS does not erase existing ones.  There is a mechanism on the GPS to erase existing waypoints, routes, and tracks.

My garmin 60 did not use a memory card, all its memory was built-in so I do not know about using mulitple memory cards.  But for a trip to quetico, I downloaded maps from Topo Canada and Topo USA.  For areas near the border, the GPS had three sources for maps: the base map, the Topo Canada map, and the Topo USA map.  The two topo maps were in different scales, so as we paddled down the border, weird displays occurred. Normally the supplemental maps overrides the basemap, but there is a way to tell the GPS to pick a particular map.  In my case, when we were on the border, I told the GPS to ignore the TOPO USA map and just use the TOPO canada map.

You probably have enough memory that you do not have to worry about using multiple memory cards while on your trip.