Not saying you can't do it with Arduino
Okay then I will say it: You can not emulate a SNES (or NES) with just an Arduino.
The Arduino is a Micro
controller with a limited amount of RAM. Both of those systems had micro
processors, video processors, and sound generators all of which were more powerful than the ATmega328.
Wrong project for an Arduino.
Now, obviously you have
waaaaaaaaay more experience than me (no sarcasm here, you have like 1000 times more posts than me), but as it seems to me what you are saying is that the limitations of an Arduino (in this instance) are:
- Sound production
- Video production
- microprocessor (which, correct me if I am wrong, is very similar to a microcontroller but it has more pins and such?)
- "power" of ATmega328
In my twisted/erroneous brain (and I will always admit that it makes mistakes), it seems that sound production is easily achieved (mp3 shields and such), video production is also easily achieved (LCDs, even RCA has been done), and the only thing that I can't think of a solution to is the last two points, based entirely on the fact that I am unsure of what you mean here.
If what you mean is not enough pins, I'd feel fairly confident saying that there are boards/controllers out there that have more pins than an ATmega328 (which I believe, without checking, is on the Arduino Mega, therefore giving the most pins of all "official" Arduino boards).
If you mean memory limit, I'd also feel fairly confident saying that there are boards/controllers out there with more memory capacity than an ATmega328.
If you mean something different entirely, then I'm just further proving myself a noob

.
you might want to go with something like a raspberry pi. Not saying you can't do it with Arduino, but it would make things much simpler to use a raspberry pi.
I don't know a whole lot about Raspberry Pi's, but I do believe it is a bit bigger as far as form factor from a microcontroller, and doesn't it also use an entirely different programming language too?