BATTERIES - Since you have a D200, make sure you have another - and another - charged - battery for that D200. Batteries for the flashes.
ISO - Don't run the ISO over 400 on the D200 - the noise will be awful. Use 200 as much as you can.
SENSOR - Make sure your sensor is clean for the shoot. If you're getting dark spots in skies, you're going to get dark spots on white dresses, or faces, the same place on every shot.
FLASH - An off-camera flash bracket like the stroboframe that allows portrait and landscape with a quick flip, some sort of mini-softbox for indirect and bounce flash - Lumiquest Promax is relatively small, cheap and works with any hot shoe flash with the Velcro strap around it. Fong Cloud, others are available. I would not use any direct camera or hot shoe flash. If you have to get any new stuff, set it up and practice so you know how it works before the day. Don't expect anything new to work right for you on the day unless you've checked it out and know it will.
Check the site - will the ceiling be low enough, light enough for bounce? Fortunately you have the LCD to check how things work. If you don't usually work with flash, make sure understand the settings and that you have the flash and camera flash settings set up in a way that works.
LENS - For me, it would be nice to have something a little wider than 35 on DX for group candids, dancing, etc. The 17-55 is the ideal lens for this job with DX. The full range is useful for a wedding.
Since others have the longer lenses, your 50 is probably fine, though I'd like to have an 85 or something for close-up candids at the reception. The 50 should give a nice quality to those shots, just not as tight.
YOUR STUFF ON THE DAY - Make a list, organize your stuff and pack it the night before. Make sure you have a way to carry it all easily in a way that it won't get in your way.
ORGANIZATION AND MANAGEMENT - The prime photog needs to make a list with the couple, and the bride's and groom's families, of what family groups they want shot. Typically it's bride with parents, groom same, both with both sets of parents, both sets of siblings, bridesmaids and groomsmen, bigger groups (danger - lighting challenge, framing challenge - not too wide a lens or you spread those people out at the sides, but the distance makes lighting a challenge). You need to scope out the time and location for the group shots in advance, figure out the lighting, have the order of the groups set so you run them all through - this takes substantial time if you have to run anybody down - skip them if they don't show. If they're outdoors, and they're facing into the sun, they'll be squinting and the contrast will be awful. Use a small enough aperture to get everybody in focus.
Get the parents or somebody in the party who seems to have organizational skills and some influence to help police the group shots, or you'll lose them. Arrange what specific key candids you want in advance and make sure you work that out with the participants. You need to insist on getting faces in the shot for these (e.g. car departure) - make them sit still a second for these or the shots may not work.
The pro's portfolios will be helpful for scoping out key shots you want and working with the couple and the families.
RELAX AND BE A PHOTOGRAPHER. If you've got your stuff organized and your shoot planned in advance, and you're familiar with your gear, you can just be happy you're not the one with the prime responsibility - maybe you can save his behind with a couple of key shots he missed. You'll see the light just right, catch a right angle or expression, an interesting juxtaposition of shapes and shadows.
Enjoy.
