Compared to...

Studio scene comparison (higher sensitivities)

As you can see the EOS 40D does a fairly good job at maintaining detail at higher sensitivities although you can clearly see the softening effect of noise reduction, you can also see chroma noise creeping in (this could be reduced using the optional 'High ISO speed noise reduction' option.

ISO 800

Canon EOS 40D Nikon D200
Pentax K10D Canon EOS 30D

ISO 1600

Canon EOS 40D Nikon D200
Pentax K10D Canon EOS 30D

ISO 3200

Canon EOS 40D Nikon D200