Vclaw
3
I think for the Nuvi you can have one map in the internal memory (named gmapsupp.img) and another on the microSD card (also named gmapsupp.img), and it will work with both of them at once.
I think it will also use a gmapprom.img file in the internal memory (note the gmapprom.img file might be used by the included / built in maps, so you might want to take a backup of it before copying over it).