Canon Digital Rebel XT Upgrade?

To be honest with you the extra two mega pixels will make virtually no difference at all. You certainly would not be able to see a difference unless you were routinely doing large posters. Even then it would be minimal. If you were to upgrade I would say you should upgrade to the 40D or 50D. These are more robust cameras and the higher frames per second could be useful for sports. They also have much better performance at higher ISO's. Other than that I would say you should put your money into a really nice fast lens like the 70-200 F2.8L. This is probably the best lens you can have for sports. It is especially good for basketball and other indoor sports. It might be a bit short for football games but unless you have $5,000 or more for a larger lens it is still your best option.