I'm fixin' to find out
I just started a project that should shed some light on the subject.
I was given a 4+ year old Dell Dimension about a year ago. Since then, the (only) fan in the machine has become obnoxiously loud and I decided to do something about it.
If y'all aren't already familiar with a common trick Dell uses in it's computers, it is that the only fan in the machines are ducted with the intention of pulling air in from the front of the case, past the vertically arranged hard drive(s), and then through the fins of the CPU heat sink. Finally, through this proprietary duct arrangement, the hot air is ducted out the back of the machine. The fact is, it must work, because they've done it for a long time, successfully. (Granted, the anemic qualities of these machines probably make this technique more able to handle the heat generated than a machine of any power).
At any rate, the factory installed fan had to go and I didn't care to find and install an identical fan into the machine, so I opted to install a 120mm into the roof of the machine. I have completely eliminated the original fan and duct, so I may find that there is a problem with airflow across he passive CPU heat sink and temps try to skyrocket.
My intention is to run the machine without the duct system for a while and see what happens. Meanwhile, I am in the process of designing and fabricating (or finding and adapting something from a commercial source) a duct system that will blow air onto the CPU, instead of draw air from it as the original duct did.
At this point, the project is in it's infancy- kindly forgive the lack of "finish" evident in the enclosed photos. That will (hopefully) come at a later time. Also, let me mention that I had to do a bit of structural surgery on the machine to facilitate placing the 120mm fan in the roof, forward of the PSU. It required that the optical drive be moved down one bay and the chassis had to be cut back.