Hey Brandonite,
You know we've all been in this together for a long time now, so please know that we're looking out for you. In this situation, you really must, must wear a tie. This isn't like the guy who asked whether or not to wear glasses to the interview. If you're nervous that you won't be comfortable the day of the interview, practice wearing a tie around your apartment for a few days, just to ease into it (so that you're not fidgeting with it throughout the whole day). Others have said that Duke is conservative, and I think they might be understating it.
Now, regarding your green suit. If it is very dark, then it might work - and you'll make a good impression by not going with the standard navy or pinstripe. If however, it is a lighter green, or it's more than a few years old, then it might be worth the investment to go out and buy yourself a new, sharp one. I'm not one with money to burn, but if you think about the cost with respect to how much you'll eventually borrow to go to Duke, the cost of a new suit is nothing in comparison - and as I said before, it'll be an investment in your future.
Besides, when you get dressed the morning of the interview, either in your green suit and tie, or a new suit, you'll feel really sharp - and therefore really confident. It'll come through in your interview and it'll all be worth it.
Oh, and one other thing. Make sure you've got a pair of shoes that match that spiffy outfit. They don't have to be new, but make sure they're not worn out.
We're all routing for you - knock 'em dead.