1. What to wear - If you live in Atlanta and the invitation says "black tie optional", take the "option" and wear a cocktail dress. There were a lot of men in tuxes but very few women in long evening gowns.
2. Err on the side of simple - If you have a question about what to wear, go for simple and elegant. Even if you are too dressed up or too dressed down, you will still look nice.
I searched "elegant simple black dress" on http://www.shopstyle.com/ and came up with these options plus more -
3. People make a party - After going to a few black tie events and to New York Fashion Week where everyone is dressed up and ready to be seen, I can tell you that glamour is a mirage. There's nothing about being dressed up or in a certain fancy place that makes anything inherently more enjoyable. Connecting with people will give the party a spark - and fancy clothes may actually get in the way of that.
The moral of the story is wear something you feel comfortable in so you can focus on the people and forget about your clothes.