I use TreeSize Pro, but they do have a free version: http://www.jam-software.com/treesize_free/

I found 120G was too small for a system drive and was having to juggle things constantly, I have no idea how you are coping with 80G. Things have been a lot easier just using a 600G VelociRaptor as the system disk and moving things that I cared about the load speed to the SSD.