When I did the side by side comparison, I didn't feel the extras were
what I needed.
However, you did say something that peeked my interest...Better
focusing and much more camera...Any specific details?
Here's the side by side, but it really doesn't spell out the difference well.
http://www.dpreview.com/reviews/compare_post.asp?method=sidebyside&cameras=canon_eos40d%2Ccanon_eos450d&show=all
40D vs. XSi
----------------
17 shot RAW buffer vs. 6
0.95x pentaprism vs. 0.87x pentamirror viewfinder (notable difference)
sRAW vs. no sRAW
6.5 fps vs. 3.5
40D can take the Canon programmable remote cable release, XSi only takes a standard cable release with a lock
rear control wheel vs. having to hold down the +
- button and move main wheel on top
exposure compensation with rear wheel is much easier than holding down the +
- button since you're already using the main top wheel to change the Av or Tv value.
manual mode is much easier since rear wheel controls apeture and top wheel controls shutter speed. With the XSi you can only change one value at a time since you need to hold down the +
- button to change the apeture value.
Even nikon users envy the Canon rear wheel.
40D has FEC button ontop that allows you to change the value without moving your eye from the viewfinder.
The 40D is designed to change shot settings by either keeping your eye in the viewfinder or by looking at the top of the camera. XSi has no top LCD readout so you're either keeping your eye in the viewfinder or looking at the very large LCD screen on the back to change settings. Using the rear LCD for shot settings uses more battery power compared the top LCD on the 40D.
40D has 9 cross type AF sensors using f5.6 or faster lenses (with is basically all Canon and Sigma lenses that are and have been in production for the past several years). XSi only has a cross type AF sensor at the center spot.
Unless you are specifically wanting a very small camera, a 40D is just a better camera with more controls, better performance, and more features. And a $300 price difference now sure isn't a lot when it used to be $500 a couple years ago.