Best to use an overlay if your GPS accepts multiple map files. That way you can update your road maps any way you like and the contours will still be there. The one I've got has contours for all the UK at 10m intervals but it's a pretty big file - about 100Mb. I also have an overlay for France and borders, and the Swiss/Italian Alps but that is spaced at 20m intervals (which is actually better in mountainous areas) and that's about 150Mb.