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I agree with the above posters that the 40D is the best choice at the moment. I bought one about 4 months ago, and I shoot with it every day. I do not regret buying the 50D one bit. The few minor updates do not really justify the cost differential. Also, 10MP is more than adequate for the stuff that I shoot. If 15MP is really that important to someone, then that person should be looking at a full-frame DSLR.

Also, I used the savings difference to buy a 50mm lens and am looking to get a 28mm lens as well.

Edit, I also wanted to mention that I have had a rebel XTI since September 2006. I would not recommend getting a Rebel or one of the lower end Nikon cameras. You will quickly find that they are too limited and will be wishing that you had upgraded to a higher-end camera.
 
Keep your 40D. The 50D is simply a 40D with more pixels crammed onto the sensor, which results in more high-ISO noise and lesser image quality.
 
I have been using the 40D for the last 1 1/2 years and was at one point considering the 50D, but after having played with one decided that it's definitely not worth the extra money. A friend has on and, wishes he had got the 40D.

Also like beatzfreak said that the Canon 60D will be out at some point, which will see video and such. So at moment 40D is pretty much the best bet, unless you have the extra money then go for 50D
 
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