Quiet - Peaceful - Silent... Serenity. The Puget Systems Serenity Pro proves that high-performance computers can still be whisper-quiet. Intel's high-efficiency Core processors are the platform for all Serenity systems, but this Pro variant offers more expansion room inside - at the cost of a larger footprint. We use the already-quiet Fractal Design Define 7 case for this, but the stock chassis fans are replaced with super-quiet Noctua models. The layout of those fans is changed as well, to cool the internal components with as little excess noise as possible. The exact noise level will depend on the specific hardware selected, but the configuration options are somewhat limited to avoid any specifically loud components.
If you want great performance combined with very quiet operation, look no further.