I've seen a couple ways to do this in the past; they mostly rely on downloading individual quads from a master KML file. This has its benefits, but the approach has never really appealed to me. I've developed another way that pulls individual tiles from existing web map sources. Beyond USGS topos, you can also load the other CalTopo map sources like USFS maps, contour lines, canadian topos, etc.

Mt Whitney Example (will pull 15MB worth of tiles when opened in Google Earth)

It's pretty straightforward. From http://caltopo.com/map.html, center the map on the area you want and click "Export to Google Earth" at the bottom of the left bar. Select the map layer(s) you want and click "Export" at the bottom right of the dialog.

If there's interest, I can also look into exporting KMZ files so that you can load custom maps onto the newer Garmin models. I don't own one of them, so I'd need someone to work with me on testing.