Although Andrew has already made its choice, I'd like to express my opinion.
Optically the 80-400VR is equal (if not best) to the praised 100-400IS. But I love Canon USM
implementation and its one touch zoom.
I tested the Sigma 50-500 for a weekend and I got a lot of vignetting at any aperture (those were slide times, so maybe OK
for 1.5 cropped digital shots).
Sigma's 80-400 OS
is not a solution as it also lacks AF
-S (or HSM in Sigma's nomenclature).
Yes, the lack of AF
-S makes the 80-400VR too slooow, specially with the cheapest Nikon bodies with CAM900 AF
module (D70, D100). With my F100 (CAM1300) there was less hunting and faster AF
and I hope that with the D2X (CAM2000) the thing almost gets solved.
Of course the 70-200VR + TC
-17 combo will deliver better optical results and the 70-200VR + TC
-20 similar ones, but what about using it in dusty environments? Bigest advantage of the 80-400 is its versatility.
Ideal solution: (D2X + 70-200VR) + (D2X + 200-400) if your budget can handle it
[Edited 2005-04-03 00:15:30]
There is a very fine line between 'hobby' and 'mental illness'.