Back in the day, I ran an NES emulator just fine on the original PowerPC laptop, the PowerBook 5300c. So NES or older (Atari, etc,) should work on any PowerPC. SNES you might need something a little faster, but not significantly so.
Connectix Virtual Game Station (a commercial Playstation emulator that was subject to a lawsuit by Sony,) officially required a G3/233, but it would run acceptably on a 604/200.