Putting a fan inside the system box impedes air flow and may make things worse.
I don't believe Charlieman's statement is speaking against cooling systems like
this GCC fan simply because the first sentence in his post talks about the benefit of "drawing air up through the top."
The original Macintosh convection cooling design was such that cooler air would come up through the bottom of the case and the warmer air would go out the top. The GCC fan shown in my photo, used on the HyperDrive, was little more than a slight modification of the original cooling concept. The GCC fan simply pulled
more cool air from the bottom of the case and out through the top. I think this was actually a better approach than how Apple later cooled the SE and SE/30, with a fan that did not sit at the top of the case. As a result, the SE series Macs allow hot air to float up to the top of the case and stay there. Even if one argues that the air inside circulates, the fact is that the top of my Mac SE/30 gets quite warm even with the fan running, meaning that a lot of hot air is staying in that pocket at the top of the case (staying there because there are no vents whatsoever at the top). Had there been vents at the top with a fan pulling air out those vents, I suspect the SE series would run cooler.
The keywords to cooling old Macs are "warmer" and "cooler," not "warm" and "cold." Because even if you have a great fan/cooling system in your old Mac, how much cooling will you really get if you are trying to use your Mac in a hot room, with all the windows closed, on a summer day where the room temperature rises to 90 degrees F? Indeed, having the A/C turned on is a recommended approach for an old Mac, regardless of whether they have a fan or not. Because remember, the original concept was to have "cooler" air come up from the bottom through the top. But if there is no "cool" air in the room to begin with, you are basically circulating hot air.
So perhaps the best solution is to keep your operating environment as cool as possible. I'm not say an old Mac would not benefit from a fan at the top of the case. But keeping the A/C turned on in your Mac room during the summer time is a must.