1. A mesh enclosure, like Antec LanBoy Air. Remove every single fan.
2. A fanless power supply, like Seasonic X-400.
3. A gaming motherboard (for heatsinks), like ASUS Maximus V Gene.
4. A 35W TDP processor, like Intel's i3 2120T or i5 2390T.
5. A passive CPU cooler like Silentmaxx
BigBlock, Thermalright HR-02, Nofan
CR-95C, Scythe Ninja 3, or SilverStone's upcoming
SST-HE02.
Add some SSD, now enjoy your solid and totally silent pc.