Garmin just changed last year from being able to do 2 units for each update, down to only 1 unit. Something to do with the license from NavTeq. People were not happy about it.
You can load mapsource and the map data onto multiple computers.
Before upgrading I would check if it's worth it. You can check the maps on Garmin's site, to see if the area you're interested it updated. The 2009 North American maps around shipping in some places, but you still can't view them on Garmin's site.
You could also check Navteq's map reporter to see if they've even updated your area of interest.