My 20D is my first digital SLR, so I have nothing to say about how it compares to the D10. Also, I only shoot in RAW, so I can't comment on JPG captures either.
I can tell you about my experience with the 20D noise characteristics though. 1600 and 3200 are actually quite usable, with a few caveats. First, I'd give them a good shot of NeatImage as a matter of course. Second, the noise is tolerable in the brighter areas of the image, but can be much worse in the shadows, as you might expect. This spread characteristic at high ISOs requires extra attaention with NeatImage. At lower ISOs, say up to 400, there is much less noise difference between bright and dark areas.
The annoying thing about the noise in dark areas of high ISO images is that there is sometimes a non-random quality to it, especially in the green channel. NeatImage can handle random noise, but faint green lines are harder to deal with. Not impossible, and if they're dark areas detail is less critical anyway, but still annoying.
Wes Baker