Overdress, generally. If your friend is promoting it, and assuming he's your age, odds are it won't be an ultra-exclusive scene (no offense - not a bad thing) so you're not going to be dealing with something way out of your normal league.
If he is promoting it, he could very well already be getting some bottles comped, I've been to similar situations.
I would avoid going with the theme too much, or even at all, unless it is literally a costume party - chicks might but I agree with whoever said black dress pants or slacks and a NICE shirt, no tie, a jacket if you have one.
A suit would work as well. NO SUNGLASSES. Jesus.
You can win if you're the best dressed/overdressed, you automatically lose if you're underdressed - a general lesson.