I did an ISO test with my 7D.
Put the camera in a dark room (basement) leave the lenscap on, close/block the viewfinder, use RAW or disable ISO long exposure/noise reduction. Choose the settings that you want (Manual mode). Take every picture for at least 10 seconds and you will come up with al list like me.
The 7D is an 18MP camera which is equal to approx. 18MB file size. (the smaller the file size, the cleaner the image is)
As you can see is ISO 160, 320, 640 the camera's sweet-spot... The golden rule is that the limit is MP/MB+1(file size) so 19MB is in the safe/clean zone and 20MB is pushing your camera. It's no math, just hard numbers.