The xxD are heads and shoulders above the xxxD cameras
IMO (and I have shot with both and decided to go the xxD route) the xxD (30D and 40D) cameras are heads and shoulders above the xxxD (350D, 400D and 450D Rebel XT, XTi and XTsi) cameras.
Although the image quality is not greatly different between any of these cameras (they all provide very good to excellent imagery), IMO, the xxD cameras handle a lot better than the smaller Rebel types and they have some features which are very nice to have and unavailable in the xxxD Rebels.
I like the two wheel control system on the xxD models and I absolutely have problems working without the topside LCD for controls which is only available on the xxD models.
The xxD cameras, while not overly large, are larger than the Rebel types. The smaller size of the Rebel may advantage when carrying/packing/storing the camera and possibly for persons with extremely small hands. However, I much prefer the larger grip on the 30D or 40D when actually using the camera. The 40D is more sure-handling, in my opinion. In all other features, the 40D is minimally equivalent and often the feature leader. This is especially true when using a larger or heavier lens. many photographers feel the need to add a battery grip to the xxD series cameras in order to bulk them up but, that is just an extra expense and something else which can go wrong. The 30D and especially the 40D cameras have such good battery life that the extra battery in the grip is not needed for general shooting.
The much shorter shutter lag and mirror blackout along with a far faster frame rate and deeper buffer makes it much easier to capture the perfect moments with the 40D - especially action sports. The 40D's fastest shutter speed is 2x that of the XTi/400D (1/8000 vs. 1/4000 sec) and has a shutter life rating 2x that of the XTi. The 40D has a pentaprism viewfinder while the XTi utilizes a pentamirror - The 30D and 40D's viewfinders are larger.
The 40D has a more-rugged stainless steel and magnesium alloy chassis - and feels more solid. It more solidly mounts to a tripod. The 40D has 1/3 stop ISO setting increments and has ISO 3200. The 40D has Kelvin white balance. The 40D has spot metering. The 40D has an easier/faster to use Quick Control Dial and Multi-Controller while the XTi uses cross key buttons. The 40D has an LCD panel on top, it also has a PC Synch port. 40D includes a heavier-duty neck strap (with red edges). Some of these feature differences are more or less important to you personally - and I doubt you will care about the neck strap difference.
The 40D has a more advanced autofocus system than the XTi AND 30D which share the same AF technology.
It seems as if the 40D is a far superior camera to both the xxxD models and the 30D. IMO, it far surpasses the xxxD Rebels but, the 30D is in itself an excellent camera which is fully capable to providing professional performance.
I have been using a refurbished 30D for over a year now with absolutely no problems. Adorama has these for $640 with free shipping http://www.adorama.com …furbished%2030D&item_no=1
Canon has a $200 rebate along with a reeduced base price for the 40D which brings the bottom line down to $940 after the initial rebate.
http://www.bhphotovideo.com …_EOS_40D_SLR_Digital.html
I would not purchase either camera with a kit lens, instead I would purchase a 17-55mm f/2.8 IS lens if money is no object or a 17-50mm f/2.8 Tamron lens if you want a very good lens (without IS however) at a much lower price. Either of these lenses would be just fine for advanced amateur or professional use.
In fact, for a wedding photographer, two refurbished 30D cameras and two 17-50mm f/2.8 Tamron lenses would be just about the same price (approximately $2,000 total) as a single 40D and 17-55mm f/2.8 IS lens and would be a better purchase... Redundancy is the name in wedding (and many other types) photography.