Just a FYI... though my GPS is a 2820, not a 660, but they work the same I think..
I backed up then deleted my GMAPPROM.IMG file and used Mapsource to select, then download my preferred mapset to the gps.. it's working fine. with no GMAPPROM.IMG and just the GMAPSUPP.IMG file on board.
Mapsource will also write to SD card.
So if you are already good at Mapsource, no need to learn MapInstall.
The map data has a lot more granularity than what the map installer gives you choice for.. this allows you to get a lot of what you want onto the 2Gb, but be advised that many towns have a tiny map associated with them, it's easy to miss selecting this tiny area when creating the map set.