Unless you get really esoteric, water cooling isn't going to be any quieter than excellent air cooling. Virtually all water cooling systems still use fans to cool the fluid in the radiator,. These can be big slow quiet fans, but my case for example (Antec SLK 3700AMB) is cooled by one big slow fan quite effectivly. Water cooling also involves a small but real noise from even good quality pumps
What water cooling can do is allow you to overclock higher than you might be able to with excellent air cooling. If you are really cranking the voltage and pushing the limits of the chip, it gets you an edge on somthing like a SLK900 with a moderate fan.
It is possible to have a water cooling system with no fans, by using huge radiators, or pumping the fliud into an underground reservoir or something.
IMO, you can get just as silent as normal water cooling with a moderately overclocked, high end system using the best HSF like a Zalman 7000, or SLK 800/900 and a case with excellent airflow and slow fans. If you want to push the absolute bleeding edge, water can be more silent than air, but for average sane overclocking even, air is still up to the task.