If you have a chance, go to a camera store, and play with each camera body. Check how the camera feels in your hand, play with the controls and go through the menu screens. Whichever camera feels more comfortable in your hands will be the camera you are more willing to shoot with. All the cameras you have listed will take fantastic images.
I'll third that one of the best improvements you can make is updating the 18-55mm lens. There is a variety of EF-S and EF medium telephoto lenses you can upgrade to. You just need to remember that if you upgrade your EF-S 18-55mm lens with another EF-S medium telephoto lens, the new EF-S medium telephoto lens will not work on a full frame camera body. The EF-S lenses only work on a crop camera body. However, any full frame EF lenses will work on a EF-S crop camera body without any problems. So you need to ask yourself first, do you want to stay in the crop body, or do you want to upgrade to full frame?
For lens upgrade, I would suggest you take your 400D camera body to a camera store, and try out the different medium telephoto lenses with your camera. Take some pictures, and examine the image quality you will get from these new lenses. See which lens feels more comfortable in your hands with your 400D. You can also try new camera and lens combos that interest you. Again, whichever new camera and lens combination feels more comfortable and "right" in your hand, will be the one you be happy shooting.