Ray, I noticed that you have a 1D MkIII which, correct me if I'm wrong, is a 1.3 crop factor body.
Add the 100-400mm L IS and you'd have 520mm f5.6 @10 fps. Add the 1.4x TC and you'd have 728mm f8 (centre point AF).
However, if you opted for the Sigma 150-500mm, you'd be looking at 650mm f6.3.
But then your 350D is a 1.6 crop factor (only 8 MP and slow fps for wildlife on the move) 640mm @ f5.6 with the 100-400mm Canon or 800mm f6.3 with the Sigma 150-500mm.
All these 1.6 specs look way better with the 7D (DIGIC IV) at 18 MP and 8 fps, outboard flash control and 1080p HD movies..... The 1D IV (Dual DIGIC IV) is 16 MP and 10 fps.............
So many numbers and decisions!!!
I don't know what will be the right decision for you, but I hope to win Gold Lotto or find a generous sponsor. Let us know which way you go.