I use Mac myself but from time to time I get asked to build silent PC workstations for friends. Here are some parts that I'd highly recommend.
Case: Antec Solo (there are many good cases out there, this one needs no modification to make it silent. It's also small, which enhances the airflow.
CPU: Intel Core I7 Quad (The faster the better)
GPU: Gigabyte GeForce 9600GT 720MHZ 1GB 2.0GHZ DDR3 (Silent since it has a passive heatsink so no modding is required)
PSU: Enermax Modu82+ (Hands down the only PSU that beats Seasonic, make sure you get at least the 625W or higher)
OCZ Vertex SSD (For your main OS)
Western Digital Caviar SE16 (Very fast drive with extremely low noise and vibration, more reliable than Samsung but just as quiet)
Scythe Ninja CPU heat sink (Although this HS can cool a quad core CPU passively I suggest attaching a very slow 120mm fan)
Exhaustfan: Noctua NF-S12-1200 (You can run it at 7 or 8 volts)
Intake fans: One or two Nexus 92mm running at 7 volts
Scythe Kaze Master (Very nice fan controller with heating sensors)
I'll leave the motherboard and ram suggestion to somebody else as these change everyday. I generally like Intel boards like the DX58SO, very stable motherboard.