some tips when choosing fan.

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RPM = Rate of fan spinning. Higher the RPM, the higher the airflow and noise.

CFM = A general measurement of airflow. The higher, the better the cooling, in theory.

Static Pressure = the amount of CFM that penetrates heatsinks or radiator fins. The higher the better performance for heatsinks.

Size = Thicker fans tend to have better static pressure.

DBA = A general noise measurement. The lower the better.

Quality = The better the quality, the more accurate the above ratings, and the longer lasting the fan.

Bearing type = This determines a lot. Here are some common ones.
-Sleeve. These are very quiet and inexpensive. However, their life span is pretty low depending on the quality. Some low quality ones have problems being mounted horizontally.
-Ball bearing. These are some of the most long lasting fans you can buy. They are also loud.
-Fluid. There are a lot of fluid bearings, such as hydro-ball bearings. These are quieter than ball bearing and longer lasting than sleeve. They tend to be unreliable with their airflow and RPMs.
-Proprietary. These are getting very common. They are odd designs made by companies. Just research them before you listen to the hype. "