a7800DS is an Atari ProSystem 7800 console emulator for the DS/DSi.
To use this emulator, you must use compatible roms with a78/bin format. Strongly recommend you use NTSC roms... PAL ones have more scanlines and will render more slowly and since the sound core is tied to scanlines, the sound will be wrong.
Most things you should expect from an emulator. Speed is excellent on the DSi and a little less great on the older DS-LITE. For the DS-LITE you can expect full speed on a very large chunk of the 7800 library. The more traditional games (think: Asteroids, Astro Blaster, Robotron, Food Fight, Centipede, Pac-Man Collection, etc) will all run great.