I have once pondered between the 400D and the 40D, but I chose the 40D and loved it.
I have recently got my hands on the older 350D (maybe not exactly the same as the 400/450D), but it cannot compare to the 40D one bit! They are absolutely two different classes.
Yes, 40D is much heavier, but it feels VERY solid. The shutter is built with more durability with dual motor mirror corking. The pop-up flash is gear-driven/damped. And I find the top LCD panel very useful, nearby dedicated buttons to change settings, as well as using the Quick-control dial or Multi-controller at the back to navigate menus instead of using the 4-directional buttons.
I find Live View useless unless at difficult angles, otherwise you'll be looking through the viewfinder. Contrast AF or not, it is just marketing gimmick and "extra" feature that doesn't or shouldn't affect your photography. 40D also offers a brighter viewfinder which is an advantage.
*I switched from a DC with manual controls to the 40D.
I took the following at a camera shop with the 70-200 F2.8L IS + my 40D. Very nice bokeh. Image was cropped and flipped in PS.