If you updated your PSP, I think that takes up a lot of space on it. It is recommended, from what I hear, that you delete the old updates. Also, if your browser says you do not have enough memory (I remember reading this somewhere, but I don't know if it was for PSP. More than likely) to visit a page, delete your cache, then try to set your cahce to zero. given the size of the memory stick, I wouldn't think that is a problem.
More than likely the first will solve your problem.
