Will a 705 map card work with an 800? Thanks for any help.

Short answer, yes. Is it a Garmin CityNavigator map? Look it up on the Garmin website which has compatibility info for all their map products.

OpenStreetMap can also be used on the Edge devices.
An advantage I found is they have off-road trails included, which are not in CityNavigator.
Instructions here: https://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin

The card will likely function, but I'd be expecting the maps to not be licensed to the 800, only the 705.