Well, late night arithmetic dyslexia has struck me down again. The increase in linear resolution moving from 8 to 10MP is 12% and not the 16% I quoted in my, now corrected, previous post.
As for the lens question, I guess the proper forum is the one you think most likely to generate the needed answer. If it's for a Canon camera then I guess this is the place but perhaps you might be better starting a new thread.
P.S. Don't let me dissuade you from buying a 40D if you want one! Mine's already on order and my pixel count won't change, though I expect them to be nicer pixels!