Those passive heatpipe cases (by Zalman) are ridiculous. With a little care, you can build an actively cooled system that falls below the ~22dBA@1m ambient levels in even the most quiet residential environments--without sacrificing performance. Here's how:
Antec P180 - the gold standard in quiet cases, but the TriCool fans need to go bye-bye
Scythe Ninja Rev B - can passively cool damn near anything in a P180
Scythe S-Flex SFF21E - just about the quietest fans you can get that start reliably at 5V; one comes with the Ninja, you'll need a second
Corsair HX520W - Whisper quiet like other Seasonic-based PSUs, but Corsair adds modular cables without the stupid 60mm fan on the M12.
Thermalright HR-03 - Get rid of that loud stock GPU cooler and put on this bad boy, which sits right in the same airflow path as the Ninja
Gigabyte P965-DQ6 - Best stock passive cooling system on a motherboard. Asus P5B Deluxe gets honorable mention.
Put the S-Flex fans in the rear and top mounts of the P180 as exhaust (preferably with AcoustiFan or similar silicone soft-mounts), undervolt them to 5V using one of several methods (Zalman Fanmate, the 5V wiring trick, NMT-2 temp-sensitive controller, etc.). Get rid of the fan in the bottom chamber. Now you have a 22dBA@1m system that can run any C2D at stock and any graphics card short of the 8800. If you want to overclock, put your optical drive in the bottom bay, put a Scythe Kama Bay without the included fan in the top three bays, and block off the front fan grill (with duct tape). The graphics card becomes a partial baffle that keeps most of the airflow going straight across the upper half of the motherboard where nearly all of your heat is being dissipated (by the Ninja, DQ6, and HR-03). Put all of your hard disks in the bottom chamber cage, remove the upper chamber cage completely, and remove any remaining I/O slot covers. Now you have a 22dBA@1m system for moderate overclocking. Add the Kama Bay fan for more cooling at a 1-2dBA@1m noise penalty.
That's essentially what you'll glean from 100 hours of reading SPCR, condensed for your convenience.