The NES Mini has about 500MB, about a CD's worth.
Enough to host every NES game.
I have it full of the better NES games, and a nice healthy library of SNES.
It can also play N64, SMS, Sega Genesis, GB/GBC/GBA.
If you set your folder directories to hold 30 games, the menu doesn't feel slow. Still feels nice and sleek.