If you want SILENT, get passive cooling.
Quiet, on the other hand, is much more feasible. As long as your system runs below the ambient noise floor of your room/environment, it will appear to be silent.
Anyway, as stated, 120mm fans are quieter than 80mm fans simply because they are bigger. They can spin slower and move the same amount of air. PSUs are normally the usual culprit in older noisy systems. Corsair and Seasonic are two companies I know to have quieter PSUs since they use a 120mm fan that is also fairly quiet.
You'll also want to optimize the airflow within your case so that you can use as few fans as possible. Negative or Positive pressure is up to you, but it's a given that more fans = more noise. Hard drives are another source of noise, if not from its idle spinning and seek noise, it's from the vibrations sent to your case. You can try to isolate/dampen them to reduce this. Along this train of thought, you may also want to dampen the fans as well by mounting them with those rubber/silicone fan mounts.
Along with the airflow thought, you may also want to seal up some holes in your case if any, that may reduce the efficiency of the airflow (for example, a gaping hole in the side panel when you're trying to direct air up to the top of the case.) As a final resort, you may try to use foam or something similar to pad the inside of your case to absorb some of the noise it produces before it leaves the case. Also, try to minimize the amount of "open vents" facing toward you, as they are the most direct path to reach your ears from the case. In my case, my PC sounds significantly louder with the P182's front door open rather than closed, even though it's running the same hardware in either case.
Edit:
Forgot about undervolting those fans. That is also another option to try, as slower fans are also quieter, although the tradeoff is moving less air, which is why you wanted to optimize airflow in the first place.
Hope this helps.
