Agree with Buckrage. I also shoot Nikon, like TCAT, but Tamron makes a good lense, in my experience, for Nikon. I imagine their Canon lenses are good for the price, too.
I also agree the lower F stop for low light conditions is important.
I never got into processing images much (my editor did that and now my wife does) but could you shoot RAW images and them crop them, if you don't end up going with a 300mm? Might allow you to get away with a lighter lens if you use a larger image.