I have not looked at modding PSPs in a while but last time I checked the hackers were looking at a tool to plug your external hdd into your psp and use that as memory.
Another thing that is useful if you are short of space is a photobank, it is a cool external hdd which allows you to plug in your memory card and write filed from the hdd to the card without a PC.
however internal memory i am not sure about, i will try to find the PSP sites I used to visit for you