Unfortnately Google Earth doesn't have an option for UTM coordinates, which in my view are the easiest to work with on a GPS.

ExpertGPS is built to actually create the waypoint using detailed USGS topographical maps - you don't have to manually enter coordinates into the GPS. Just find the place on the topo map, select the waypoint tool, and then click to create the waypoint. You'll be presented with a dialog box to name it, give it symbols, etc. Then you connect the GPS to the PC and simply upload selected waypoints from the PC to the GPS. You can also download waypoints & routes from the GPS to your PC. Very cool!