I'm building a new system and I want to keep it as quiet as I can without degrading performance. I do plan on doing some moderate overclocking. I am planning on using an all-in-one CPU liquid cooler. Am I better off getting one with 3 120mm fans for the extra radiator surface area or will a smaller radiator with 2 140mm fans still keep the system plenty cool and quieter?
If you only plan to only cool your CPU with the AIO cooler, I would say 2 140mm fans will keep your CPU plenty cool and be quieter then 3 120mm fans. However if you plan to cool your GPU as well, you will want the 3 120mm radiator. Regardless of the 2 140mm or 3 120mm radiator, the kind of fans used on it will make all the difference.
You can very easily get 3 120mm fans that are considerably quieter then 2 140mm fans, however having less fans will make it easier for keeping things quiet. From personal experience I have found that very rarely will any cooler come with high quality fans. I usually buy fans regardless if the cooler comes with them or not, that way I can make sure the cooler will be as quiet as possible while still being able to provide good performance.
For radiator fans, and case fans in general, there are 3 things to keep in mind when choosing a fan - fan speed, noise level and static pressure. Check out this video for an explanation about static pressure vs air flow and why static pressure is important - Static Pressure vs. High Airflow Fans As Fast As Possible
I have personally had extremely good results from Noctua fans, they have a low sound level, good performance and a long life. For a 140mm radiator fan I would recommend this fan - Noctua NF-A14 PWM, if you went with the 3 120mm radiator I would still recommend the Noctua brand - Noctua NF-F12 PWM
Also, it is important to keep in mind that ultimately what is important is not the amount of noise that the fan makes but the amount of noise you hear. Things like where the fan radiator is placed, or what it is placed in, can go a long way in reducing the amount of noise you here. One such method would be to place the radiator in a box lined with noise suppressing foam as a way to decrease the noise that you hear.
It is also important that your computer fans are setup so the air flows as efficiently as possible, such as fans in the front are intake fans and the fans in the back and top are exhaust fans. Another thing to keep in mind if your bios offers this feature is to set the fans to variable speed, this allows the fans to only increase their speed (noise) when the computer needs them to.