Whatever gear you use, make sure you have a backup body.
For lenses, you want them to be as fast as they can be in their class. So for zooms, that's f/2.8 constant if possible. Many wedding togs' also use a twin-body twin-primes setup. The most canonical example of this is the 35/85L combination, although there are people who do 24/135L, 24/85L etc. A flash is also useful.
For your 400D, I can't think of a better zoom lens to do a wedding than a 17-55 f/2.8 IS.
+1, a dual body setup will not only serve as a backup but save a lot of hassle of swapping lenses.
@Mankini, still no pictures of shang yet O_O.


When are you getting your Mark IV?




