Loafer:
I honestly don't know the answer to that question, because I have always had the same cartography loaded on my computer for use with other GPSRs that don't come pre-loaded.
I do know that the 2820, and all the other GPSRs that come with all of Europe pre-loaded, include a CD in the box entitled "Trip and Waypoint Manager'. But, I don't know how effective this is for route planning on the PC, for someone who has never, ever had maps installed on their PC before. (NB: - to those of you who have maps and MapSource installed on the PC already, installing Trip and Waypoint Manager will have no effect, because the CD will already recognize that there is a 'full install' of maps on the computer).
To get a good answer to your question, we need to hear from someone who bought a GPSR that came with preinstalled maps, and loaded Trip and Waypoint manager - never having had MapSource installed before. Anyone out there who can help???
Al:
I would not recommend dicking around with the map preload in the 'Garmin' folder in the 2820 or the Nav III. In theory - strictly in theory - what you say is correct, but, the GPSR was designed on the assumption that the full map load for the region (e.g. all of Europe) would always be present. You might get very flaky results if you delete the preload and install a partial load. Also, be aware that the 2820 / Nav III comes with CN 8 NT loaded on it, not CN 8. You need to have the NT build to support all sorts of newer features, such as TMC, postal code lookup, stuff like that. If you were to load the 'Plain Jane' CN 8 onto a 2820 or Nav III, the results would be unsatisfactory. Also, if you install a partial load of CN 8 in addition to the preload of CN 8 NT, you will also have unsatisfactory results.
2820's and Nav III's are designed to work with the NT version of CN cartography only. You may load other 'non-NT' map products - for example, City Select South Africa, or a topo map, but you should not load any non-NT City Navigator products.
Michael