The museum that I work for got a federal grant that matched funds, so we were able to build a new building. We are using a system called spacesaver (http://www.spacesaver.ca/)I know that it is a Canadian company, but there has to be an American equivilant. I have seen several cheaper versions of spacesaver systems, but if you are going with a system that is on wheels as oposed to in a track, make sure you have solid wheels, and not air filled. If you have air filled the tires will not be able to sustain the weight that you are going to load onto them.
I would suggest checking out a system that can be installed in your current storage loacation. An issue that comes up from that, however, is where do you put the artifacts for the retrofit.
hope that helps.