I got the 3.5 inch in my auto R33... love it.. the noise hits you at the start but then you get used to it within a week, tops two.
Actually it isint that loud at all. Its only droney if you drive around slowly, above 80 KM+ on the freeway, its quiet as a bee! A mate who has a manual R34 says his car is way louder than mine.
Go for it, get the full 3.5 system done... go turbo back, you will love the difference.
I recommend JJR exhaust for 900 bucks with split dump pipe.