Hi all - I am looking at getting my first DSLR. I am looking at the Canon EOS XSi. I have a concern about lenses though. If I purchase the XSi and start buying lenses for it, down the road if I upgrade to something nicer like a full-frame Canon 7D or 50D, would I be able to use those lenses with the "higher end" DSLR cameras? It seems like lenses are a good investment, that can outlast the camera frame. I just don't want to make a bad investment however. Thank you.