For something like that, fill flash or full-on lighting is your best friend. I've used a 550EX on a flash bracket with good results. But if you can set up some studio lights and fire them with a Pocket Wizard, that's even better; more power and faster recycle times. When lighting the portraits, have the main light on the side of the groom, and the fill on the bride's side, so that the white dress is getting less light than the dark skin of the groom. That will give you a bit of breathing room exposure-wise.