PSP supports upto 4GB memory sticks, currently 2GB is the max you can buy, but Sony is set to release 4 and 8GB sticks in the next few months (althought the PSP will only support the 4GB stick, they will be very expensive but we should see a price drop in the 2GB stick at the time of release)
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Also Does the PSP only support Memory Stick Duo, or regular memory stick..
And what's the difference??
A regular stick is twice as big as a Duo, and wont fit inside the PSP.