I just got a used PSP and need to get a memory stick. The PSP is not here yet, so I have some time to shop around. I thought I needed to get something that was duo or pro duo, but now I see some posts about people using SD cards? What's the deal, are there different formats that are compatible with PSP? I'm just looking for something that I can pick up at bestbuy/target/whatever for a reasonable price in the 16GB - 32GB range.