As a wildlife photographer, you may want to consider that your telephoto lens will not appear to be quite as long as with the 1.6 factor. I think the 1D Mk III is a 1.3 factor, the 1DS MkIII a 1.0 factor.
Just a thought. I love my 20d, and the wifes 30D does not quite get me to replace the 20D, now the 40D? I don't know yet, some improved features, but is it enough to change? I have not decided that yet. Basically the same IQ. The 1D Mk III, well for the price of it, even though I salivate, would buy me the 40D, 50D, 60D, if they follow that sequence. So I may try not to spend it all at one time, spread out will be easier and have newer camera bodies as I go. Actually if I were to spend it all at once, the 500 f4, L IS sure is appealing. 