I have a Sigma 500 f4.5, a D200 and a D70 and have been happy with the results. I have to say that when using a Kenko 1.4x the Sigma has a tendency to "hunt" and is very difficult to use with a 2x (mainly because the view finder becomes very dark). The raw files from the D200 are stunning and beat the D70 every time, but that isn't to say that I don't rate the D70. I tend to carry both bodies with me at all times.
Have a look at these images of mine from birdforum taken using a mixture of set-ups.
http://www.birdforum.net/gallery/sho...9/ppuser/18900
http://www.birdforum.net/gallery/sho...2/ppuser/18900
http://www.birdforum.net/gallery/sho...3/ppuser/18900
http://www.birdforum.net/gallery/sho...9/ppuser/18900
http://www.birdforum.net/gallery/sho...5/ppuser/18900
http://www.birdforum.net/gallery/sho...6/ppuser/18900