It really boils down to how much you want image stabilisation, if it is a priority then the Canon range is the way to go if not stay with the Nikon I think. The 40D is only just available but the price is just beginning to drop a little. The IQ of the 30D and 40D supposed to be very similar with the 40D a little better and the 40D having a setting to help highlights a little. The 40D is 30% faster focusing than the 30D with all the focusing points being the cross type so better for tracking birds in flight. Very few reviews of the 40D out at the moment but those I have read look good. The Nikon D300 looks like its going to be a brilliant camera but at a higher price. Gosh this is making my head spin, but I would be loath to run two systems so would probably go with what you have and buy a second hand Nikon none VR or Sigma 500 and save for the D300. Just glad I already have a Canon set up!
