Also, if you haven't already, read Bob Atkins' Guide to Canon EOS Speedlite System over at Photo.net before you invest in any flash equipment for the first time. This includes a pretty good overview of how flashes work as well as detailed explanations of TTL, A-TTL, E-TTL, and E-TTL II. Understanding this will help you decide whether you want to go Canon or 3rd party, and which 3rd party flashes you might want to get.