http://www.silentpcreview.com/

And check the forums they're quite friendly:

http://www.silentpcreview.com/forums/

To get an efficient PC first thing you'll need an efficient power supply (PSU) and that means building your own PC since no brand name PC uses a really good PSU. By good here means high power efficiency meaning both lower elctrivity bill and less useless heat to evacuate hence more silent PC so every percent count :).