QUICK! Going to party tonight and taking candids. Help please!

Hi everyone,

I've done some really cool studio stuff with my Canon 50D and my 430EX II, but I'm going to a party tonight and I was wondering if anyone had suggestions for how to make the most of my camera equipment. It's going to be mostly outdoors while the sun is setting. I have a Tamron 17-50mm f2.8 and the Canon 85mm f1.8. Will the f2.8 be fast enough? Should I even use the flash? If so, should I use second curtain sync for any reason? These are the types of things I'm wondering. Any tips would be awesome. Thanks!