For your examination, on the left I've placed a photograph taken at my cabin in Red Feather Lakes. This shot was made at dawn, in very low light as the morning sun was poking through the trees. The original image was shot in raw at ISO 1600. I converted it to 16 bit tiff, processed it with Noise Ninja and did a little post processing on the noise reduced file for comparison. What you'll see here is that the noise level of the 50D at ISO 1600 on what is a very dynamic light condition with a lot of shadow and very bright light in spots. Noise reduction via Noise Ninja was set for defaults. I did some sharpening and detail enhancment to the final tiff file. Everything was then converted to jpg for the web. Personally, I think you can pretty much not worry about noise up to ISO1600 on this body and even a properly exposed ISO 3200 shot will clean up to make a usable 11 x 14 print.
Exif: Canon EOS 50D
ISO 1600
1/320 sec
f/4.5
100mm focal length
EF 100-400 f/4.5-5.6 L IS USM