Hello Jim, I had a 350d for a while before it took a swim in the ocean
, the camera performed great on almost everything i targeted.
I now have a 400d and find it even better, higher resoution, that wouldnt make much of a difference but helps in larger print size, i heard by only an inch or two, 350d, 3456 x 2304, and the 400d 3888 x 2592 Max resolution.
Sensor cleaner, a good idea, and also the dust removal software, for those stubborn motes, you basically take a pic of a flat white wall, or screen and the camera records it and uses it to remove the dust from the image through the software, i haven't tried it yet, but it seems like a great idea, saves you from trying to clone out dust on a decent pic.
If you had a 350d, it wouldn't really be worth upgrading to the 400d, in my opinion, but i would buy the 400d in a flash over the 350d if i was looking at purchasing a dig-cam.
Regarding noise, have a look around the net at comparisons, both cameras are almost equal in high iso noise results, at least from the tests i have seen, as far as lower sensitivity, i am not too sure, i will dig out some pics taken from my backyard wth the 350d and match the settings and take the same image with the 400d.
Hope this has helped.
Cheers
Trevor.