Chuck - I bought Metroguide North America with my recent purchase of a Vista Hcx. Given the reality of the limitations of viewing the map on a small screen, I like the maps. I don't know how the data in Metroguide compares with City Navigator, but Metroguide has all of the information and more that I need from a street map in a hand-held gpsr and it was less expensive.
Metroduide loads as a map layer within MapSource. The license agreement restricts usage to one computer. But, I didn't see any limitations on then loading it from that computer to more than one gpsr. You don't have to "unlock" the unit to load it.
To load the maps into the gpsr, you first have to create a file of maps in MapSource - use the map tool to select the map parcels that you want to use. If you intend to periodically change the maps, I would suggest saving the various files so that you don't have to recreate them. Limit the size of the file to the amount of available memory on your gpsr - MapSource will advise you about the size of the file as you build it. Once the file is created, send it to the gpsr. That process erases the temporary memory, i.e. previous map data that you have sent to the unit, regardless of the amount of memory in use and the size of the new file. This process does not disturb the base map or saved waypoints, tracks and routes.
With a 1 gb card, you probably have enough memory to load most of the country. But, it will take awhile to build that file and quite awhile to transfer all of that data to your gpsr.
I don't know for certain, but I suspect the memory card is only good for map data. Waypoints, etc. store on the unit's "hard drive".