There was a proof of concept NDS emulator made for PSP, I wouldn't bother with it though.
As for PSX yes its possible, I wrote a guide on it!!! Its pretty good as well! See it up in the stickies.
When you make the eboot the PSX ISO is compressed slightly typically by 200mb so a 700mb disc becomes about 500mb, there's no way to make this even better unless you're prepared to take content out of the ISO such as music or videos.