Nice and thanks - I managed to find a site that clearly addressed this.
Your GPS will offer up a very long list of options under “Map Datum.” What's important here is that the Map Datum option you select for your GPS matches the Map Datum on the map - the individual topo - that you're working with. New GPSs come from the factory defaulted to WGS 84 (World Geodetic System 1984), but because virtually every "store bought" USGS topo for the United States uses the NAD 27 Map Datum - I've yet to encounter one that doesn't - select NAD 27 (North American Datum 1927) if you're using these off-the-shelf maps. If your GPS offers different breakdowns of NAD 27, choose NAD 27 CONUS (North American Datum 1927 Continental United Sates) if you're in the lower 48 and, if you're in Alaska, select NAD 27 ALASKA. (If you're in Alaska but your GPS doesn't offer the NAD 27 ALASKA option, just stick with NAD 27.)