Silent PC Review ( http://www.silentpcreview.com ) occasionally reviews complete systems. Last year they measured an i7 machine from Puget Systems at 11dB in their anechoic room ( http://www.silentpcreview.com/Puget_Serenity_Pro ) which is the best I've seen for an off-the-shelf PC.

I haven't used a Puget machine myself, but I've relied on Silent PC Review's component recommendations for nearly a decade, and never been disappointed.