Here is the (Almost) ultimate guide to emulation on your PSP, written by me a couple of days ago.

If you have Firmware 1.5, skip down below.

For 2.0 users, you must first downgrade to 1.5 before any emulation becomes possible. Go here for instructions how to downgrade. Make sure you read it carefully, I'm not responsible for any damage you cause.

K, now for emulators. 1.5 requires the KXploit patch to be applied to any EBOOTs, most authors are packaging their applications already in 1.5 format. This consists of two folder that go into the GAME folder: [Name here], and [NameHere]%. The only downfall of this exploit is that it creates icons in the Memory Stick menu that say "Corrupt Data", that can be annoying. But don't fear, SeiPSPTool will fix these. Go here for that.

Now, don't you need the emulators?

Emulators that run great, if not perfectly:

MSX
------
fMSX (With ram fix by Artemio from Junkerhq.net to play Snatcher)

You gotta put the BIOS files in a folder called "BIOS" inside the MSX directory.

NES
-------
Nester J

SNES
-------
Snes9x
I use the following settings for all of my games, to get them running at a nice speed.

Sound Quality: 22khz
Frame Skip: 3
PSP Clock: 333mhz
Frame Size: 4:3

Neo Geo CD
---------------
NeoCD

Neo Geo CD requires you mount the ISO in Daemon, then copy the files from the CD directory to a ZIP file, or a seperate folder and put it in the NeoCD directory. Then inside the game directory, put a folder called "neocd.mp3" and put all mp3s in there. They must end in a track number to work. Remember to set the Clock Speed to 333mhz. ENJOY YOUR METAL SLUG!