We just moved into a 50 year old house and we want to add insulation. The original insulation is thin and looks as if it will disintegrate if you touch it. Should we remove the old stuff or just add new insulation on top it?

Removeng the old insulation is not nessesary.
Unless it is wet or showes signs of a pest problem.

blown in (cellulose) insulation would work great if you are not using the attic for storage.

If fiberglass insulation is used without removing the old stuff use "un-faced" insulation. if you do decide to remove the old stuff then use "faced" insulation. put the vapor barior (face) toward the heat source (down) hope this helps