Well depends on what your doing, personally ive kept all my case fans due to the fact my new card aint got the waterblock on, but ive seen people with just two 120mm fans which if you get quality ones that spin slow are fairly quite, with a decent setup youll find your
PSU is probably the loudest thing.
Edit: oh and if you do go watercooling, its worth getting a quality one, not the likes of a globelwin thingy!