Btw.. very nice pictures you made with your canon and lenses.
Especially of Yosemite, I've been there too :)
The pitures of El Malpais National Monument are also very great.
I also had the 400D for a few days... but now I have the 350D. I compared both cameras and my experience is that the 400D has more noise at 1600 than the 350D.
This because of that the 400D has 10MP instead of 8MP. With 10MP each pixel has to be smaller..this give more noise and the dynamic range will be a bit less that the 350D.
That's one of the reasons that Canon keep it at 8MP in the 30D!