Meanwhile, I think this memory storage capacity issue is probably THE major difference between the 550 and the more recent high end Garmin models.
Nay, nay, Mr Bull. I've solved this little issue, and posted about it here: Click for zumo550 enlightenment
You can load the entire North America mapset and more!! Just be sure to put it onto the SD card (8GB recommended if you also wan to load 1000 mp3's) not the Garmin's internal memory.