ProSystem is the best emulator of Atari 7800 game console, running on Windows system. Here is a port on GP2X of the version 1.1 that ZX-81 had previously ported to PSP.

Changes:

– Merge diff from ProSystem v1.2
– Improve speed
– Improve emulation accuracy
– Add MMU hack stuff
– Add option to display frame rate
– New speed limiter function
– Improve volume handler
– Linked with my patched version of SDL
– New background image
– Bug fix in file requester

http://zx81.zx81.free.fr/serendipity/index.php?/archives/180-GP2X-7800-Atari-7800-emulator-for-GP2X-v1.0.2.html