As requested, I have made some 100% crops at ISO1600 and ISO3200 from both the 40D and 20D. All were shot in RAW and converted with DPP and then cropped and slightly sharpened with Photoshop CS3. the lens used was Sigma 30mm f1.4 DC. Shutter speed and aperture were matched exactly the same on both cameras.
The first shot is from 40D at ISO 1600
The next shot is from 20D at ISO 1600
The following shot is from 40D at ISO 3200
The next one is from 20D at ISO3200
You can see the Canon EOS 40D full review article here.