Normally you would set higher ISO if you would like to capture the surrounding ambient. For 400D I would say the comfortable setting (with acceptable noise level) would be ISO400, and ISO800 if stretching it. Also, remember to watch out the corners if you're using the wide end of the 16-35. Don't put people too close to the edges or they will be distorted.
17-40 is kinda the "standard" lens that photographers use... i spoke to a wedding photographer whom i came across at a friend's wedding ( and another wedding my cousin's, different photographer but said the same thing) that this was the lens they use most often for weddings with flash.
i might not be using canon, but i'm using nikon kit lens to shoot weddings and events. although it's 18-55 vs your 16-35, taking a few steps forward to get the reach of 55mm isnt much of a problem you know