I don't if this is a very good answer.
As stated above, it depends on the PC case and its components. However, we can do a simple calculation to approximate how much airflow we will need. For example, lets assume that we have a fairly powerful PC that is putting out 300watts of heat. Lets also say that we want our PC to operate at an average temperature of 32 Celsius at steady state (constant heat input, constant temp, constant airflow).
So if we operate at steady state we know that the heat generated by the PC must be removed at the same rate by the airflow, or
Qgenerated = Qremoved.
We know the amount of heat being generated, but we need to find the heat removed. The amount of heat removed will be the mass flow rate of the air multiplied by its specific enthalpy, or
Qrem = mass flow rate (M) x change in enthalpy (dh).
Since there are so significant changes in pressure this equation can be stated as;
Qrem = density (p) x volumetric flow rate (V) x specific heat of air (cp) x change in temperature (dT)
The volumetric flow rate, V, is what we are solving for. The change in temperature is a function of what we want our PC's average temperature to be at (32C) and the ambient air we are blowing into the case (25C). So solving for V we get,
V = Qrem / (p x Cp x dT)
Plugging in all our values we get,
V = 300 J/s / [1.184 kg/m^3 x 1012 J/kg*K * (32-25C)] = 0.0358 m^3/s
or
75.9 CFM.
So to keep our 300 watt space heater at 32 C we only need about 76 CFM of airflow. There are obviously a lot of assumptions that are made with this model but it gives you an idea of the minimum amount of airflow required to keep your PC cool. If you have a good airflow setup I would probably multiply this number by 1.5-2 to give yourself a good factor of safety. For anyone that has a calculator handy, what happens to the required airflow when you want to cool the PC by another 2 degrees? How about 4 and 6 degrees?