Second neuro's welcome back.

I have not used the 400D, but neuro's observations seem spot on to me. I have a 50D and it is a great body. If you can get one in almost new condition for a good price, and that is your current budget I would not hesitate. Not to take away from the 7D, but keep in mind that the 50D was the top of the APS-C sensor line until the 7D came out. Although I do not use mine as much as I once did, it still performs very well.