If you need a silent heatsink, and I do mean silent, the Scythe Zipang (SCZP-1000) is the next best thing to a passive block of aluminum (the only truly silent option). Equipped with a 138mm wide fan that spins at a lazy 1000RPM, the Scythe Zipang heatsink stands a moderate 112mm tall.
FULL STORY @ FROSTYTECH (http://www.frostytech.com/articleview.cfm?articleID=2248)