On nüvi 1xxx units it is possible to fit the whole of North America on the device if you delete unused files (especially voices). You'll have about 90MB remaining afterward. The full-size JCV file for full-screen units needs to be on the MicroSD. It won't fit internally with the map. Come 2012.40 or 2013.10 at latest the map will no longer completely fit on a 2GB device, no matter what files are removed. Use MapInstall (Included with BaseCamp) to manually transfer portions of the map to MicroSD.
For me, when this happens my upgrade path will be to use MapInstall to place just my home state on the internal memory of the device. This is to prevent a potential "No detail maps. The nüvi cannot navigate without them" error that may appear with no detail map present in internal memory. I will then install all of North America to the MicroSD card. The JCV file I will move from external memory to internal memory. If that doesn't work, then I guess I move Canada and Mexico to the MicroSD and leave the US on internal memory.