These are 100% crops, taken at different ISO settings. These were taken indoors, with very poor lighting conditions, flash off. The camera was standing on a solid support, using a 2 seconds timer, so no chance of camera shake.
The photos were taken at "large JPG, highest quality" The crops were made using Canon's own software, at "highest quality", so I would expect there to be little or no loss from the original.
The whole photo looks like this, the white rectangle is the crop:
From left to right, you can see crops taken from photos taken at ISO 100
, ISO 800
and ISO 1600
My own opinion (and I'm certainly not a professional) is that ISO 800 is usable for most purposes, while ISO 1600 is downright unusable.
Just out of curiosity, here is a crop of the same subject, taken at the same time, and at ISO 1600, using a Nikon D40, with NEF/RAW and Nikon's own software to crop (still no flash, camera standing on a solid support with 10 sec timer):
For full details on the settings of each photo, please see here