At first I want to support carstene1ns statement, that it’s technically possible to run EasyRPG on the PSP but currently the port is too unpolished and not many people would be happy.
We hope to offer a nice port someday though because theres actually some demand.
Just adding some technical details 
64 MB is not thaaaat bad. The Wii has 88 MB but you don’t get 100% of this memory because the operating system wants some memory, too (well or the IOS in the case of Wii, has no OS in that sense). As said Yume Nikki, Ib & co. will probably run because they are memory friendly but forget any other games (needs a per-game testing).
The problem is that we use 100% software rendering (the only hardware accelerated step is “copy to screen” because upscaling in software to screen size is a waste of time). This makes porting much easier because you only have to write custom code for the screen-copy step, remaining code is always the same.
Software rendering is fast enough for RPG_RT because you work on a 320x240 surface, which is a laughable size for modern systems (your PC screen usually has 1920x1080 or more, just multiply it and you see a slight numerical difference :D)
We actually did huge performance improvements in our renderer because the Nintendo 3DS has really awful hardware and we had a hard time to reach 60 FPS on normal maps. This moved the PSP port actually in a more likely shape again, because of its slow CPU. One year before a PSP port was basicly just lagging like crazy. There was many years ago a PSP port but it was only so fast because the compatibility of EasyRPG was in it’s beginning, so many stuff just wasn’t emulated at all (less accurate = faster obviously).
The PSP still has one annoying hardware limitation: The CPU is slow and only has one core, this means that we have to do the audio decoding/playback on the same core as the game logic & rendering. So depending on which type of audio you play the framerate decreases or the audio stutters. On PSVita and the 3DS we simply handle the audio code on a 2nd CPU core to not influence the framerate.