Well, I normally start off by offering new members to the forum a "warm welcome". I guess that is even more appropriate in your case!
I can't begin to address the question of how well any of your proposed kit will work at -20°C and falling. Hopefully others will be able to pick that one up.
I upgraded from the 400D to the 40D and have never regretted it. The 40D has a number of bells and whistles that the 400D lacks and which I find very handy for my astrophotography but, to be brutally honest, the ultimate image quality between the two models isn't that different. Look at build quality and ease of use and the 40D wins hands down, however. In the end it's an emotion thing and for me the 40D just feels right. Pick up both models in a shop and play with them until the assistant starts fidgeting and then play with them some more!
I like your choice of lenses, though I would be tempted to swap the EF-S 17-85mm f/4-5.6 IS USM out and go for the EF 24-105mm f/4L IS USM. From Canon's blurb for that lens we have the "L-series is Canon's flagship professional lens range, combining outstanding image performance and ultimate operability with dust and moisture resistant construction". Might that lens be more suitable in the Antarctic?
As for the EF 70-200mm f/4L USM, it's optically excellent and short of going for the IS version, at considerably greater expense, I doubt that there's a better lens at that zoom range and aperture.
By the way, don't forget to factor in Canon's cashback promotion
running until the end of the year.
P.S. It's just rumour
at the moment but it's possible that the 400D will be superseded (450D?) early next year.