Hi,
EDIT to post:- here is a page that explains all you need to know about what I wrote below specifically for the Garmin 800. Google is your friend etc…….
http://www.justgoride.co.uk/Content/Articles/Item.aspx?id=119
What map formats does the Garmin 800 support?
I use my Motorola Defy (and previously a HTC Desire) as my bike computer. There is a program called Mobile Atlas Creator which allows you to grab open source maps from the Interweb and create maps for your device.
I use RunGPS and have created both OS maps which change scale as you zoom in from 1:50000 to 1:25000 etc. I have checked the program and it does allow to create them in the format Garmin Custom Map (KMZ).
If the 800 could use these maps it would save you buying the maps or having to faff about with scanning and calibrating the maps.
It does take a bit of time to create your maps and there is a bit of trial and error to get them to a usable size etc. But if you can waste time on STW forums you can equally waste it setting up your maps 😉
An OS Map in 1:50, 1:25 and road atlas covering about a 30 miles from my house in all directions is about 230mb in size.
Regards