I 2nd shot an inside wedding with the 50mm 1.8G once, D3100 without a flash, and got a lot of darn fine pics. The church was VERY well lit for the shots I was taking, though... As almost everything was personal, no groups, I shot mostly at f2.8, bumping to f4 occasionally. Other settings, from what I can remember, was auto ISO up to 800 (remember, D3100), minimum shutter speed of 1/60... and I think center weighted metering, but can't be sure on that.
If ya know your camera and the venue, you'll do just fine. I showed up a little early, while the main photog was shooting some group pics, and got a feel for the areas where I would be shooting, and that's all it took. Since you've described this as a "party", I'd imagine the lighting might be a bit more challenging, though, so for the flash, only you'll know for sure. have it along, just in case, and if there are going to be action shots (i.e. dancing, etc.) think about boosting that minimum shutter speed a bit!
Good luck!!