Seeing as space is money on the internet or atleast it seams that at times, Mitch have you ever thought of say archiving past topics, say 6 months old or less, into a compressed file to save space. The only thing is that the topics should be searchable. Compressing older topics though would save space. In the future you probably will have to do something about it though if your not doing anything now.
This is all given that you haven't done this already and that you have control over file management.