The chaps over at PCPerspective have taken three massive heatpipe heatsinks from Thermalright and tested them in passive, pseudo-passive, and active cooling modes. The contenders include the Thermalright HR-01, the XP-120, and the Ultra-120.  A great article for all of you trying to balance cooling performance with noise output in your own system. Here's a taster:

We have now confirmed what was suspected all along - a little airflow can make a big difference when it comes to keeping components cool. In passive mode the HR-01's relatively large surface area makes up for minimal convection airflow and produces good cooling results. Increasing airflow thru the HR-01 with the Thermalright 120mm Fan Duct dramatically improves performance. Using a fan to force air thru the much smaller surface area of the XP-120 produces similar results to the HR-01. And the Ultra-120 demonstrates what can be done when both surface area and airflow are increased for outstanding cooling performance. Each of the three Thermalright heatsinks offers certain advantages for specific cooling configurations.

HR-01 in passive cooling mode: No fan, means no added noise; a potentially silent cooling solution. But the hardware configuration, layout, and case ventilation will be critical to successful operation.

HR-01 in pseudo-passive, ducted fan made: Coupling the HR-01 to an existing 120mm case fan offers the advantages of significantly better performance without adding a dedicated CPU heatsink fan, and the waste heat is exhausted out the back of the case (instead of just stirred around inside).

XP-120 with 120mm fan of your choice: As we've seen, the larger, tower-style Thermalright coolers offer better cooling performance than the older XP-120 (along with being a lot easier to install). However, the XP-120 is still a very good cooler and it does have the advantage of providing better airflow to motherboard components in the CPU socket area. For example: the XP-120 is much better suited to cooling the surrounding components on the Asus P5N32-SLI deluxe motherboard than either the HR01 or Ultra-120.

Ultra-120 with 120mm fan of your choice: The Ultra-120 is still king of the hill when you want the best performance air-cooling can provide. (That's why it was recently awarded the PC Perspective Gold Award!)

If you are looking for a virtually silent CPU cooling solution that doesn't add another fan to your system, but instead makes use of an existing rear case fan, the HR-01 used with the Thermalright Fan Duct is a great combination! If you don't mind having a dedicated fan on the CPU heatsink, then the low prophile XP-120 or tower style Ultra-120 are two more excellent choices. I would like to thank our good friends at Thermalright for making this comparison possible - thank you guys!