I have recently started my own wedding photography business after being an assistant for 3 years.
Your kit sounds very ample indeed. You have a wide range of lenses good enough to get what you need, the most important is a wide angle for group shots but your 18-55 Kit will do that and your 55-200mm is good for informal shots and close ups from the back of the church. The 50mm prime will be good for close portraits but you know what your doing there and the 18-55mm will be good again for macro (Cake and Rings).
You would benefit from a Speed-light and the SB-400 will do the job but the internal flash would do just fine, i recommend turning it down a couple of notches in the in-camera menu as it can be harsh or make a diffuser at home to stick over it so it's a touch softer.
I strongly recommend a 2nd battery as you'll be snapping loads and you don't want it to die on you, a tripod for formal shots as you want them crisp and a second memory card to back everything up just in case.
You may want to invest in a Cokin P-Series filter kit to use so you can add some effect to the arty shots or you can do that in post-production if money is tight (You may find them handy with your flower shots in future though?).
Have a chat with the vicar/priest or registrar before hand to see what you can photograph and what you can't and i recommend casing the location of the wedding and the reception venue at the time the wedding/reception will be beforehand to see the best place to take photos this is also handy as you'll know where you can take snaps if it rains!!
Dress smart, be punctual and polite and you will glide through the day, don't be scared to ask the groomsmen to help you where possible, that's their job!
Good Luck!