hey katie,

i don't think neither 50 or 120 can replace your 80. reading my own thread, maybe they can if used together.
in your case, i'd get the cheaper one (so you can get started) and wait for an 80 to come along.
i tend to do my "closer" stuff with an extensiontube, since neither 80 nor 150 focus close enough.
even works for close portraits on the 150.

maarten.