I don't know what your credit looks like but if the numbers seen in this thread hold true you could probably go in and trade it now and use the difference for your down payment and probably not have to come out of pocket for anything. The closer you get to the end of the year the better the deals are going to get as they try to pump up their numbers.
Also, with a new car you are eligible for their special financing offers that rarely are extended to used cars.
Credit and trade value are going to be what "drive" a new deal.
My credit is pretty decent and I bet I could cut that interest rate by a lot. As I said previously, I wouldn't be able to do anything until February at the earliest and I want to be more financially secure before I do a trade-in. However, I would love to do a trade-in and get another car.