ISO 6400 is achieved in the 40D by choosing ISO 3200 and -1 EV of exposure compensation, then pushing it a stop during RAW processing. Actually, ISO 3200 in the 40D is really ISO 1600 and -1 EV manipulated in firmware — it's why it says "H" instead of 3200. Anyhow, it worked out okay.
You can see some banding in the shadow areas in the shot above. Shadow areas were rendered poorly using this digital push process. I came away from this experiment believing that black-and-white conversion was the way to go.
The shot above shows banding, too, but it's sorta kinda useable. If you printed it fairly small it wouldn't be as noticeable. And as long as you stay away from the shadows, the 40D does pretty well considering it's being pushed two stops past its limit.