There is a great gba emulator, gpsp 9. (for phat psp)
gpsp kai 3.2 test 8 (or something like that, for slim.)
homebrew games:
"BOXHEAD" - http://pspbrew.com/forum/viewtopic.php? ... 097983aa6e
Theres DOOM
Super Mario War
And others.
2GB memory sticks are enough, you can get one for $30 or so.
Some games you might play may be 800MB, but there are usually ripped versions of the game that are less than 400MB.
And if all else fails, but the games that are too big to fit on your ms.