Matt (matt_xyz) and I have been taking about the ISO performance of the 40D. I've only had the camera a few days but I was impressed by the low noise when using high ISO settings, and shooting JPEGs.
Matt pointed me to a review where they showed that the 40D is actually slightly noisier than the 20D, when shooting RAW. A very interesting link Matt.
Today I went out and took some shots in RAW. My initial impression is that the noise is in some way different rather than worse. Hard to be exact, but I certainly prefer the images from the 40D at ISO 1000 and 1250 than the 20D at 800.
What is evident is the greater detail that shows through, which to me is far more important.
Here's a few examples:
Here's an ISO 1000 shot, full frame.
Here's a cropped version:
I regard that as a heavy crop, yet look at the amount of detail on the bird.
Here's an ISO 1250 example:
Again this is a large cropped image, The bird is as far away from me as the blue tit was.
I find these very pleasing noise levels. Yes noise reduction has been done, but at a lower level than I would do with an ISO 800 version from my 20D.
Another 1250 image:
And another:
Now a ISO 1000 one:
The light levels were not that good for any of these, as you can tell by looking at the shutter speeds and apertures I used with these high ISO shots.
If the noise is greater, it's of a type and level that to me is very pleasing to the eye, I would certainly rate these as good if not better than any ISO 800 I've taken with the 20D.
Another difference I'm noticing, the 40D seems to meter more accurately than the 20D. I would always start with -1/3 stop exposure compensation with the 20D. I'm getting the same results with the 40D with no compensation. All the above were taken with no exposure compensation, except the blue tit, which I applied just -1/3 stop as the sun had just come out and I feared the white could be burnt out. With the 20D, I'd be applying at least -2/3 stops in the same situation. Even the Great Tit is OK at 0 Compensation, something I would rarely if ever get right if I used that setting on the 20D.