My son just got a PSP for Xmas. New to PSP I have a question. I was helping him with music on it and it says it is only complatible with Memory Stick Duo. So how come when I search for additional memory to buy for the PSP I get much more hits for Memory Stick Pro or Memory Stick Pro Duo, than just Memory Stick Duo? What is the difference? Do all of these work in the PSP? A short tutorial would be very helpful. Thanks.