When picking fans for WCing three things matter:
Size: It has to fit the rad.
CFM: Cubic Feet per Minute, ie., how much air the fan will move.
Pressure: Usually listed in inches or milimeters of water.
RPM (Revolutions Per Minute) has no bearing on fan performance. It's a useless spec that means nothing.
Note that, generally, the more air a fan moves, the louder it will sound.
That's the easy part!P-Q Curve
A quick look at the above will show you that the more resistance to it's air flow a fan sees, the less air it will move. Every model rad has a different resistance to airflow, generally depending on three things, thickness, the spacing of the fins, and the air flow (more air flow = more resistance). Further, the performance of all rads (how much heat they will remove) is dependent on how much air flows through them.
So you have to pick a rad that will remove the amount of heat you want to remove, determine how much air flow the rad needs to do that, and then choose a fan that will deliver the required air flow at the resistance of the rad at that airflow.
And, buy the way, you have to do the same thing with pumps (they have P-Q curves too).
Hope that helps........