Partly depends upon climate too. I put 2 PC fans (and bug mesh) in my main 10yo cab last year because I'm in a tropical/hot/humid area now, and it was getting pretty warm in there (and bugs too). I was worried something might blow up
Now it runs a lot cooler, esp in the monitor (27" Sony PVM) bit. I'm amazed at how warm the metal cover over the top fan (monitor bit) gets from the hot air being pushed past it.
I was also putting a fan in at the bottom I needed a cover. So I pulled the metal back wall off an old/dead PC power power supply (literally cut it off with tin snips), tidied it up (no sharp edges), used the PSU's fan grill for my new fan (with blue LED light!) and adapted the attached PC power input socket to my cab, so that now it plugs into the wall using a regular PC power cable!
The cab looks much more professional now. With the recycled power/fan plate ensemble mounted on the inside of the rear vent, there are no rough cuts to see (I made it pretty neat anyway). In addition to looking and being cooler, this means I don't need a power adapter anymore as the wall plugs are also different in my cab's new, tropical location (now I just use a local PC power cable). Fortunately voltages are the same at ~220V.
Finally, if you open the door at the front of the cab, the bright blue LED light has some weirdly mesmerising time-tunnel effect on you as you peer into the cab's otherwise inky depths...
This whole project cost me basically nothing as almost all the parts were recycled, including one of the 90cm fans (the top one). Used up some spare wire and molex connectors. I spent a couple of dollars (maybe even 3 or 4) on the bottom fan with the pretty blue LED light.
If fan noise is a potential issue for you, then could try to use larger fans (eg 120cm) as they rotate slower while moving more air (ie both less noisy and more efficient). I only used 90cm fans as they fitted neatly into my existing vent holes and other arrangements.