ProSystem is the best emulator of Atari 7800 game console, running on Windows system.
It has been written by Greg Stanton,
see http://home.comcast.net/~gscottstanton for details.
Here is a port on GP2X-Wiz of the version i've previously ported to PSP.
Many new features have been added compared to GP2X-F100 version,
such as cheat support, new background images, text/cheat editor etc ...
You can find it here : http://zx81.zx81.free.fr/