I have the Rebel XT -- In addition to the kit lens (18 - 55 mm, F3.5-5.6 EF-S lens ), I bought a Quantaray (Ritz camera brand and I wish I had stuck with a Canon lens!) telephoto/zoom 70-300mm f/4-5.6 lens -- it is not the best (slow), wish I could afford a faster telephoto, or at least learn enough to think I deserve one!
I also bought the 50mm f/1.8 -- the reason I got it was not the distance so much as the kit lens covers 50 mm but it was for the speed of the lens. It is great for nice portrait shots -- gives better bokeh (the blurred background). The faster the lens (smaller f number) the better they perform in low light conditions too. I like the prime lenses and will probably buy the 85 mm f/1.8 too. But what I really want is one of those great white barrelled telephotos for sports photography. There is a cheap one for @ $1200 that I might have to try before football season is over.
There are a couple of sites online that actually let you rent a lens. That way you can decide if the lens does what you want before you shell out the bucks for it. I am thinking about looking around to see if they would actually rent one of the really fast super-telephotos.
AS far as Canon or Nikon -- I don't thnkthere is much difference. It all depends on where you find the best deal. If you have a friend with a DSLR, might want to get the same camera so you can trade lenses occasionally.