I have a Magellan Meridian GPS (discontinued model that was the base model of the Meridian series) which takes the Secure Digital memory cards and uses Magellan MapSend Topo USA maps. I bought the GPS unit with a 32MB Secure Digital card and loaded in four 8MB regions; all of the Chicago area, all of Wisconsin except a SW portion, northeastern Minnesota (including the Twin Cities) and the western half of the UP. This got me around to most of the places I go in a year.
I recently bought a 512MB Secure Digital card ($33 after rebate) and the new Magellan MapSend Topo 3D USA ($75). I've just started playing with it, but I can now load up to 230MB regions. The technology just keeps getting better. This is really fun. I can now load all of Wisconsin, the eastern half of Minnesota, all of the UP, the western part of lower Michigan and northern portions of Iowa, Illinois and Indiana with room to spare for thousands of routes and waypoints combined. On planned trips to Arkansaw and Wyoming this year I will be able to put in all my routes and the areas I am going.
The new Topo 3D maps are much better than the original. They use the USGS 30 meter DEM, a grid of elevation points 98.43 feet apart, for the entire continental USA.
Magellan recommends up to the 512MB Secure Digital card for the Meridian series, but a tech at Thales said that even the 1GB cards will work. Amazing.