This site has instructions for copying maps to an edge 800, including prep of the memory card: Garmin Edge 800 | Ride With GPS Help
. It is very slow to copy large files directly to the memory card while it is in the Edge 800, you should use a USB card reader instead.
You'll have to use maps that are Garmin compatible. You can check OpenStreetMap on the web to see if it contains the kind of detail you are looking for. I got my Canada map from here: OSM Maps - Maps for Garmin GPS devices
But since I rarely venture too far from home I've been generating my own maps for just my province using data from here http://download.geofabrik.de/north-america.html
. Generating your own OpenStreetMaps isn't too difficult, though it requires downloading some programs and setting up some folders.
There are some free topo maps for the northwest US and parts of western Canada, but I wasn't able to find anything similar for the great lakes area. If you are looking to buy maps, there should be some options out there. Some of the more detailed maps are large and you would use a desktop application to upload a subset of them to your device.
You won't be able to load Google Earth data onto a garmin.