Dave, don't worry too much about the camera to begin with, unless you have money to burn, in which case get a new 7D.
Instead, spend as little as possible (used) on any of the bodies mentioned so far, they're all good, and instead get the best lens you can. I have both 40D and 50D bodies, both bought second hand for half the original price. In 3-4 years time they'll both be almost obsolete and worthless (exaggerating) but the lens you'll still be using in 10 years or more, assuming you find you like it. If not you'll still get back 60-70% of the cost of a decent new lens, and more if you buy used in good condition. If you find you're still enjoying photography in a year or so then upgrade to a better body if you find it lacking.
In almost all cases it won't be the camera that's lacking, but the user. It's just a fact that any of the cameras mentioned can out-perform the skills of most people that buy them! at least for the first year or so.