20D vs. 450D dark frame
Above images are 100% crops from the center of the image:

Based on sensitivity results (same lens and camera settings shooting at a white wall) the 20D is about 20% more sensitive than the 450D which is close to the pixel size difference between the two cameras. I compared the Dark frame from each camera (at room temp of 75 deg F). The 20D dark was 5 min at ISO 1600. The 450D was 6 min at ISO 1600 (which is 20% longer than the 20D to make up for the sensitivity difference). The Darks were taken as RAW and then converted in Image Plus with Digital Development breakpoint set to 3000 and then saved as jpg.

Histogram Results: (Mean and Standard Deviation)

--------------20D---=---450D

LUMINOSITY
--Mean--------12.3---------9.3
--S.D.---------18-----------16.4
RED
--Mean--------16.2---------10.8
--S.D.---------30.6---------24.4
GREEN
--Mean--------9.91---------8.12
--S.D.---------20.3---------19.6
BLUE
--Mean--------15-----------11.8
--S.D.---------27-----------25.4

The results show that the 450D has about 25% less noise than the 20D