There are two options:
- Load maps to the internal memory of the unit: the capacity is 850MB. It may not seem much, but you will be surprised how much it holds. For example, I have loaded my Edge 705 with City Navigator & Topo for all of New England + NY for less than 400MB. Make sure you leave some space for your data: log files, routes, etc.
- Otherwise, add an micro SD card to the unit: 4GB should be enough and 8GB should be plenty. With this option, you will be able to load the full map sets for both City Navigator & topo for the US.
In both cases, it is better to purchase the maps on disc or download from Garmin to install from the computer. You can pick & choose to load the entire map set, or portions of it.
There is also the option of 450t, which has tho topo maps already loaded. You then have all the remaining 850MB for City Navigator, trip log history, routes, & other data.