2hr movie will look decent at 300-350mb or so. Maybe 400mb so it will fit on one 1GB MS Duo Pro.
Speed does not matter for playing videos. The bottle neck WILL be the 'card reader' or the reader in the PSP itself. If you buy a 'fast' card reader you will see some speed difference with faster memory cards in TRANSFERING it to your MS Duo Pro, otherwise NO.
Koing