Pretty much no question that the Zalman 7000 series is the king of quiet cooling right now. Thermalright SLK series is next best at quiet cooling, although performs better with bothersome loud fans.
SilentPCReview of Zalman 7000
Lots of people are overclocking with these things running at 5-7v. You just need decent case airflow.
Basic proven silent pc recipe:
1 Case with big slow exhaust, good airflow at the front
1 120mm fan PSU like Fortron or Zalman or SeaSonic
1 Zalman 7000 or SLK800/900 with quiet 80-92mm fan