Games can be PORTED, but that's hard work, and doesn't come out too well. Take Vice City Stories and Liberty City Stories as examples - they're not good on the PS2, which they were ported to.
The PSP and PS2 processors are so different in power and scope that you cannot hope to emulate either system on the other - first because of the processing COST of running an emulator, and second because of the various calls the systems regularly perform during gameplay. Then there's the issue of the Emotion Engine Chip - good luck soldering THAT to your PSP and coding an emulator to use it. In fact, good luck doing anything other than holding down paper with your PSP afterwards.
I've always compared the PSP power to something early-PS2 ish. Of course it could play PS1 games, but I'm not so sure about PS2 titles. If you were wanting to emulate certain games, just make sure theres a PSP copy, and mayby you could buy it?
PSP 2002 PRO-B8 Yellow PAL NZ
Now playing: Nothing Much really, just hoping around games