When Jeff Atwood built his first HTPC, he "had to turn the volume up to 11 just to drown out the noise." For the next few months, he "immersed [himself] in the world of silent PC enthusiasts [SPCR]."
Jeff says now every PC he builds is "optimized for performance and low noise from the very beginning. Building a high-powered rig that's so quiet your wife can't tell when it's turned on or off -- now that's an accomplishment! It's a bona-fide engineering challenge... In the process, I've learned quite a few things about building quiet PCs. I'd like to share them with you, so you can avoid making the mistakes I did." His short article is an excellent summary of SPCR-esque approaches and techniques. Discuss this article in the SPCR Forums.