Comparison of the exact same sensor exposure in the Canon 20D at ISOs "100" and "1600". Both are pushed in these crops to an exposure index of ISO 10,000. Images are RAW data blackpointed, interpolated, white-balanced, and 2.2-gamma-adjusted. The noisey image on the top is the ISO 100; 1600 is on the bottom.

Image softness is due to a lack of any sharpening of the RAW whatsoever, and it's a 100% crop of a super-WA zoom, so it is softer than usual.