I have a few suggestions:
1) Research at www.silentpcreview.com They have good tests on quiet components.
2) Pick a quiet case. The Antec SOLO is a great one that I have personal experience with. It has insulated side panels, baffled intakes, and air filters. If you will install a gaming graphic card, keep the length at 10.2" or less. More is possible with a simple tin snip mod to the drive cage. Or, consider the P180, P182, p183 series.
3) Get a good oem cpu cooler which uses large(120mm+) slow turning fans. Most noise will come from fan noise. I used a thermalright ultra 120 extreme. It fit,...just barely.
4) Consider the new clarkdale 32nm cpu's. They run cooler and will run most things well that don't require quad cores. If you can use the integrated graphics, then so much the better.
5) Don't skimp on psu quality or sizing. A premium PSU will be more efficient, keeping the fan speed down.
Corsair, and Seasonic are good places to start.