For macro and telephoto shooting your XT or the 20D make the most sense because these cameras are like putting a 1.6x teleconverter on your lens that does not cost anything in lens speed.
The 5D does not add any frames per second to your capablilities, although that is seldom an advantage for the type of shooting you do. Im most cases for they tpype of shooting you do it is better to get one perfectly timed shot than a whole lot of shots at 5 or 8 frames per second that do not quite capture the right moment. The 5D is a wonderful camera, but it is not going to assist you with the type of shooting you do.
The 1D is weather sealed, has the most frames per second and has the best auto-focus system. Telephoto and macro lens performance is better than on the 5D, but not as good as the 20D. It is also much bigger and heavier than either the 5D or 20D if you have to carry it around in a pack to locate your nature photography subjects. It is a good choice if you are doing a lot of action sports photography.
The 20D gets the most out of your macro and telephoto lenses and has a respectable frames per second (if you still consider that a factor). It is not as well weather sealed as the 1D, but many of us have used this camera outdoors in inclement weather by taking the precaution of using a cover to reduce moisture. Incidentally, it can take quite a bit of moisture without damage, mine has had a quick dip in a river, been covered with wet snow and been soaked in rain and mist without any problems. Auto focus has very similar capabilities to the 5D, but is not quite as good as the 1D. You can improve autofocus capablility by pre choosing one sensor, especially if you choose the centre one.
You already have the XT. It does not cost you anything to upgrade, it has the same telephoto and macro advantages of the 20D. It is even better than the 20D for being compact and light weight. Auto focus is not quite as good as the 20D or 5D, but can be improved by just using the centre sensor. Frames per second are not quite as good as the 20D or 1D, but you can work around this by timing your shot perfectly. This just takes practice.
In any case, I believe it is your lens selection, not the camera that is holding you back. You want to do macro photography, but do not have a true macro lens. You will be shocked if you see the image quality your XT can deliver if you put an EF 100 mm macro lens on it or the 90 and 105 mm equivalents from Tamron or Sigma. You do have to learn to use a tripod, remote release and mirror lock-up if you want to maximize macro photo quality.
Your other limitation is your telephoto lens capablilty. Putting a 2x teleconverter on a 70-300 mm lens is not something many of us would recommend if you want quality telephoto images. Either the Sigma 50-500 or Tamron 200-500 lens would be reasonably priced choices to get you the most telephoto performance at a reasonable price. I prefer the Sigma because it has full time manual focus override, which is important if you are trying to focus on birds or animals that are behind brush. If you will not shoot from a tripod, the Canon 100-400 IS is the best choice, but it is much more expensive.
Also, get the best lenses available for the camera you have. Buying lenses for a camera you may never get in the future does not make any sense. Do not be afraid to get EF-S lenses if they are the best for your current needs. For example, the 60 mm f/2.8 EF-S macro lens may be the most economical lens to get you into quality macro photography. It will not fit on a 1D or 5D, but most of us are never going to need a larger format than EF-S. Be practical, not a dreamer.