So long as you are not the primary photographer for the wedding, and you're just snapping some additional shots in areas where the pro photographers are not present, I think you'll be okay.
I have three pieces of advice:
1) Get a hot-shoe flash! Being a Canon man, I'm not familiar with Nikon's line of Speedlites, but I assume they have similar stuff to the Canons that I have. A reasonably powerful hot-shoe flash, with a diffuser, is an absolute must for good indoor pics. Not only will it light up darn subjects, but most hot-shoe flashes also have powerful supplemental infrared focus-assist lights, allowing your camera to auto-focus in much darker areas that it could on its own. It's also a good idea to learn how to do bounce flash, and learn how to use a bounce card; these are fairly simple techniques that will help you to reduce or eliminate glare spots in your flash pics. A good hot-shoe flash will cost you between $100 and $200.
2) Concentrate! Most peoples' biggest problem with photography is that they simply point the camera in a general direction and click the button (I am guilty of this myself, even after 10 years of shooting SLR cameras). But to get better pics, you have to actually look through the viewfinder and concentrate on the framing of the shot. You don't have to stand still for 10 minutes, but a second or two of actually looking at the details of the frame, as if you were critiquing a print or someone elses digital file, will often allow you to see ways that you can re-frame to make the shot look better.
3) Don't be afraid to shoot, and shoot, and shoot, all day long. It's digital; you can always delete bad shots later, when you download to your computer, but you can't go back and re-shot a shot you missed. A crummy pic of an important moment is better than no pic at all; of course, you'll get more good pics with practice and fewer bad pics, but the best way to learn something is to do it, so click away! You'll learn as you go, especially when you review your pics later.