The XP-120 has a minimum thermal resistance of 0.167. In order to "get there," you need to push a certain airflow rate through its fins (in CFM). Anything above the required CFM will not improve thermal resistance of the cooler, nor will it cool down the CPU (for that reason) any more than at the "optimum thermal resistance."
However, the XP-120 was also thoughtfully designed to promote airflow around the hot motherboard components (mosfets, chipset -- even VGA card). It is with these components that pushing more air through the cooler than necessary to provide maximum CPU cooling will still pay off -- if that's what you want to do. Further, building a restrictive motherboard duct of foam-board or lexan to restrict CPU fan exhaust (low volume, high airflow) around those components and duct the air immediately from the back of the case -- will provide even greater advantage, but you must be willing to take the time to build and install the ducting.
All of the fans mentioned above -- I note the Silverstone, Sunon, YS-Tech and especially Panaflo models -- should do the job for cooling the CPU.
The ducting of the motherboard also serves to deaden fan noise. In my case, I chose a Delta fan (lighter than the Panaflo) which spins up to 3,700 rpm offering an airflow rate of 142 CFM. It only takes between 95 CFM and 120 CFM to maximize CPU cooling, and many of these fans will provide some of that CFM range by spinning around 2,400rpm.
What I did with the Delta proves something people looking for noiselessness in fans overlook with high-airflow/high-rpm models: at lower speeds, they can be just as quiet as fans made to run at those speeds. So I hooked up my Delta to the motherboard CPU fan-header, ducted the motherboard, installed Alfredo Comparetti's "SpeedFan," and set the program to hold the fan speed at around 2,600 rpm below a threshold that is close to my "load" CPU temperature. As it reaches that threshold, it spins up to its maximum 3,700 rpm. As the CPU drops below the threshold, it spins down again to its 2,600rpm setting.
This configuration lowered my chipset temperature by some 8F to 10F degrees at either idle or load CPU usage.
One more thing. I was disappointed with YS-Tech's 120x38mm offering. I do not believe it really provides the rated maximum airflow of around 110 CFM.
I must have tested some ten models of fans before settling on the Delta. Many here would favor Panaflo fans, and I agree that they are faithful to their air-flow spec while holding the noise level down. But the noise level of my Delta at the same rpm is about the same as the Panaflo. Also, I think the Panaflo model I looked at was a bit heavier than the Delta, while the lightest of all 120x38mm fans was the YS-Tech.
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