The GPS uses its own set of maps. Those have to be purchased from Garmin. The topo maps kinda suck, the data's kinda old and the POIs are almost useless. I have CityNavigator on there, roads are all up-to-date and it's got TONS of POIs (Hotels, restaurants, gas stations, shopping, auto repair, banks/ATMs, airports, etc.
EasyGPS is a utility to get data to/from the GPS. EasyGPS has the ability to display tracks/waypoints/routes. On the Mac, LoadMyTracks is a simple utility to get data in/out. It doesn't display tracks/etc in it, but once the data's on the computer Google Earth will read that.
GPSBabel is a great utility for converting GPS data files. Say you get some goofy magellan-specific tracklog of a place you want to ride. Throw it into GPSBabel and tell it to convert it to a GPX, save that on the desktop and load that into your GPS (or Google Earth).
Google Earth *can* talk to a GPS, but there's much better options out there. For displaying and navigating saved tracklogs, Google Earth is the way to go.
As for the data files, either go GPX or KML. Those two are "universal", in that any GPS utility worth having will read them. GPX is xml data, pretty easy to read in Notepad or any other text editor. KML has a few more features, but at the expense of being more difficult to manually manipulate.