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Dr_Jones, how do you plan to use it and how flexible is your budget? What camera are you moving up from?
If you can afford the 7D and you're going to photograph birds and wildlife, then the 7D is the way to go. If you're into scenics, portraits and general purpose, than a used 5D might be the best choice. If either of those is beyond your budget, then a used 40D or 50D would be a good choice.
The 40D will do a good job and you need to include at least one good L-series lens in your budget, no matter which body you buy. Because sensor technology is redefining Moore's Law, I think you need to buy the latest body technology that you can afford (with at least one L) and plan on staying with it for a while. If you start with 2-year old technology, then you'll be wanting to trade up sooner.
Full frame is great. I've got the 5D2 and the 7D. The IQ of a cropped bird with the 5D2 equals the equal-sized crop of the 7D (it might be slightly better in fine detail actually). The advantage of a 1.6 crop-sensor is in tracking birds with the single-point AF. With FF the bird is much smaller in the viewfinder such that it's harder to keep the focus point on the desired part of the bird. The focal length of the lens is NOT increased, with roughly the same number of pixels on the subject as taking a FF image and cropping it more severely down to the same size subject. AF is no issue with static subjects. The 7D has more "convenience" features, a faster burst rate and more AF modes.
FF is better for wide-angle images and the large sensor gives you more pixels to work with for portraits, archetecture, etc. Of coure, if you get a FF, then most non-lens will not do. You'll see softness as you're tempted to make larger and larger images.
Anyway, all the cameras mentioned are great. The high ISO performance of the 5D and 7D open up possibilities that you won't get with the 40D or even the 50D. The 7D is also fast handling and aimed at sport and wildlife photographers.
Dave
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