I don't know about Atari, but for other emulators, there are quite a few listed in HandheldAttitude
(http://www.yetanotherhomepage.com/j7xx/
). Just click Software, Games, and scroll down to the Emulator section.
I only tried Yame, which runs very well on my j728. But the screen refresh is so bad it isn't very playable
(hardware's fault, not emulator's
)
I do use the same emulator on a PPC tho. hp and NEC need to wake up and give us a new model with new screen