If I am not mistaken, I believe it can go as high as 25,000, which I doubt I would ever use. If I can get clean shots at ISO 6400 or slightly higher, I would be more than happy.
The Ricoh GR line of cameras are a street shooter's dream. Small, compact, excellent menus which are easily navigated, and the lens is super sharp. Plus, the camera is built like a small tank.