Calculate the effects of cooling fans on a system?
There is no easy way to calculate that.  The "real" way is to generate a 3-D CAD model of your system, complete with accurate heat sources, and simulate it.  That particular field is called "Computational Fluid Dynamics".  

There are so many things that must be taken into account including the turbulence of the air, the color of the components, the type of alloy used for the heat sink, and of course the actual airflow.  

The software that does this modeling is expensive.  At least US$10,000.  For the average person it is easier to just try the fan and see what happens.  Get some good thermocouples or at least a non-contact IR thermometer.