You know, when I first saw this thread I thought it said "How do I allocate more money to Poser6" and I thought to myself, well Meterman we're all trying to figure that one out, if you find an answer to it, let us all know :) And then I saw it said Memory...Well, I hope you can figure out that problem too! :) 
I have that problem with Poser 6 a lot than with 5. The problem with defrag to me is it takes forever to do. My brother gave me this holiday an external 80 gig drive that looks like a book so I'm thinking of using that for backing up my Runtime as mentioned above and maybe that will free up a ton of space. Then I'll have to buy another one I'm sure. 
Off the subject a bit, one thing I did notice was when I sorted through like my "Pose" folder, I started putting the folders into catagory folders. As in if I have a group of character files, I placed them into a folder I created in the Poser Pose directory and so on for clothing and shoes etc. This seemed to cut back on the time it took for Poser to start up...maybe it was a fluke but after I "cleaned up" my runtime a little bit, reorganized it, Poser did load much faster.