Below is a shot from my 20D at ISO 1600. It looks OK, until you crop.
You can really see the noise there.
This next shot was taken at ISO 3200 on my 50D. The crop on each pictures is the same (640 X 472).
This crop takes a much smaller portion of the overall picture, yet there is less noise than in the photo above. Awesome. I need to remember to use the higher ISO levels when I need to freeze motion.