The 350d has got an inferior focus system (digic II?) to other camera's such as 400 and then a stage further with 450d I think, not sure about 40d but I am sure it will have a similar or more advanced processor than the latter.
I have always been a little underwhelmed with my Canon, when I had a Nikon 8700 I could outperform my Canon 350d in some area's especially when cropping, and this includes tripod shots etc!
When I look on various sigma lens photo threads on other forums (great example for the 70 - 300 APO on a certain forum) I can often see the difference in shots made between a 350d and other cameras too, and a click on exif info more often than not confirms my suspicions!
My advice - read steves digicams on the camera you are looking at, familiarize yourself with the way it works, then take your camera, lens and memory to a Jesops store or local independent and have a play around and report back here!
I am sure that is a fair test and will help you make the best choice!
Oh and I would love a 450d for one main reason - I find it unbelievable that I have to pull the camera away from my face and mess around to adjust ISO, which is surly a much more important function than the stupid quick print nonsense they assigned to a dedicated button
Stupid Canon!
Olly.