I have a Canon 450 and have the 18-55 "kit" lens that came with the body and a 70-300 for sports/wildlife photography. I find this a very acceptable combination but I would agree that having an 18-70 to bridge the gap might be better.
Please remember though, that however handy zoom lenses may be, "prime" lenses (i.e. non-zoom lenses with a fixed focal length e.g. 50mm) will always give you better quality photographs. My ambition, the bank manager, my wife and my newly pregnant daughter permitting, is to get a couple of prime lenses, probably a 50mm and a 300 mm.