Well, the 360 just dropped to 350, so its already one helluva deal. However, since you waited this long and think* you want it just for Halo 3, you might as well wait until release and see if there is any "Buy a 360 and Halo 3 and get a free controller!" type deal.
*I said think because I know you will find more things you want to play on it once you get it.