There seems to be alot of people here confusing between the types of memory cards supported in PSP. i'll make a summary and comments about these cards.

currently, there are 3 types of media that can be used in PSP:
1) Memory stick pro duo (MSPD)
2) Memory stick Micro (generally known as M2)
3) MicroSDHC

Memory stick pro duo
it is the default memory media for PSP, hence it is the most compatible media for PSP

- fastest reading and writing speeds in all three cards
- read: 15mb/s
- write: 10mb/s
- no lag in all games. be it CSO or ISO
- MagicGate suppoted
- price wise, compare all three cards, MSPD having the highest price.



Memory stick Micro
a newer type of card that developped by sony for use in newer sony devices like cellphones. having a size of only 1/3 of a MSPD. in order to use it in a PSP, you need to convert the card to MSPD via a small adapter.

- average speed.
- read: 8mb/s
- write: 2mb/s
- no lag in ISO games, noticeable lag in heavy CSO games, SLOW in saving game.
- MagicGate supported
- price wise, it is cheaper than MSPD, more pricey than MicroSDHC.



MicroSDHC
MicroSD is currently the smallest removable flash memory card using secure digital format (SD). it is widely used in lotsa electronic devices other than Sony. as for MicroSDHC, it simply means MicroSD High Capacity. all microSD having capacity larger than 4gb, it is a MicroSDHC card. in order to use it in a PSP, you need to convert the card to MSPD via a small adapter.

- average speed
- read: 8mb/s
- write: 8mb/s
- no lag in ISO games, noticeable lag in heavy CSO games, fast in saving game.
- NON-MagicGate supported
- price wise, it is the cheapest card around
- there are some compatibilty issues with PSP. not every adapter can support microSDHC. some only support MicroSD. please check the adapter upon purchase.



* all mentioned memory card prices are subject to change, please survey for latest pricing. all cards that taken into comparation are 8gb, sandisk branded.

What is MagicGate?
MagicGate is a copy-protection technology introduced by Sony in 1999 as part of the Secure Digital Music Initiative (SDMI). It works by encrypting the content on the device and using MagicGate chips in both the storage device and the reader to enforce control over how files are copied. (extracted from wikipedia)

it is actually not so important for us in our country as we generally don't buy stuff from Sony online stores. you will need a MagicGate supported card to access sony stores. it is just a gimmick from Sony.

BUT, there are alot of imitations of Sony's MSPD around the market. you can use this MagicGate status to check whether is the card original or it is an imitation. a genuine card will denitely support MagicGate.

To check the status of the card, simply press "triangle" in the XMB (ie, game, music, videos etc) and press for "information".

What is CSO and ISO?
CSO simply means compressed ISO, it is just like a Zip file. where you compressed the ISO into a smaller size file with a new extention .cso . playing a CSO game will require your PSP to decompress the CSO real time, hence it requires fast clock speed and fast reading from flash memory.

VERDICT:
out of these cards that i have ever used. i would say the MSPD is the best card for PSP. althou both M2 and MicroSDHC can both perform very well on PSP gaming. but you can enjoy the fastest usb transferring speed.

but in the long run, as concern of the development of expand memory sizes in the upcoming future, i expect the MSPD and M2's prices will still on high side while MicroSDHC can have a significant lower price. if you are pursuing usb transferring performance and worry about the compaibilty issues of microSD and M2, then the MSPD is the best card for you. if not, i would strongly recommend MicroSDHC. it is cheap and good.

hope this helps clear your doubts in memory media.