I'm looking at upgrading my body at some point over the summer. Right now I have a Rebel XTi with a Canon 50mm /1.8 and a Sigma 70-200mm /2.8. I have been mainly looking at a used 5D or a new 40D. I shoot mostly in low light conditions (Theatres, Concert Halls) so noise is a big decision factor to me. For that reason I ruled out a 50D as everything I have read has criticized it for noise at high ISO levels. So I'm stuck between the 40D and 5D, any advice? Thanks!