dogsby, bearing in mind I am on a mac, so YMMV.
I was also really stumped with this. Basically, dead easy. when i installed basecamp it alse installed a program called Garmin mapinstall (IIRC, not at home right now). So, with a 4gb micro SD in the 800, and with it connected the computer, start mapinstall, choose the SD card rather the 800 at the first drop down, then select the tiles you want to install. Voila!
As i had plenty of space on the SD card, easiest way to select the whole country was to zoom out of the map in mapinstall until you can see all the tiles, then drag over whole map, hit the button, and bob’s your uncle.
HTH!
Kev