Not all xbox 360s can be modded right now I think, it depends how new it is. It requires taking the xbox apart, connecting the DVD drive to your computer and flashing the firmware. It doesn't cost anything to do yourself.
The you need to download an emulator disc with games and burn it to a DVD.
This will let you play snes and nes games.
Or you could just buy a snes and games off craigslist for like $40 or so.
If you really just want to mod the xbox 360 then check out the xbox 360 forum on afterdawn.com
'01 Bullitt #0054