There are several things to take into consideration when it comes to case
fans, and as you have found out, it can be quite confusing.
Pretty much any fan type is going to work on your side panel. First, you
have to decide if you want a "ball bearings" or a "sleeve bearings" type
of fan. They are indeed even more types, but these two are the most usual.
This article on the "Frosty Tech" website discusses the two.
"Ball bearings Vs. Sleeve bearings"
Bottom line is that a fan with sleeve bearings will be slightly quieter,
but won't, in general, last as long as a fan with ball bearings. I use
sleeve bearing fans because they do seem quieter, and even though I have
my computers running all of the time, I have never had one fail.
The next things to look at are air flow, measured in "cubic feet per minute"
or CFM, and noise level, indicated in adjusted decibels or dBa. The higher
the CFM the better, the lower the dBa the better, but of course, in general,
there is a direct relationship between the two, that is, the more air flow,
the more noise.
If the computer is in a very quiet location, or you are using it for a media
computer, you might consider a low dBA fan, but if it is in a typical household
or office environment, go for the higher CFM.
Something often overlooked is the type of power connector used. There are
"3 pin," which attach to your motherboard, or sometimes off the power
supply, and "4 pin," which attach to one of the hard drive type power
come off the power supply.
You will want to look on your motherboard for a connector marked "case fan,"
and if that has already been used, you will want to get a 4 pin connector.
Some fans come with both styles of connectors.
Any additional cooling is good for a computer, and a 120mm fan will do great.
You will want to make sure you have wire cover for it as well. This fan on
Newegg, is a good compromise of noise and air moving, is a 4 pin 120mm sleeve
bearing fan, and comes with a wire cover as well.
"SILVERSTONE FN121 120mm Case Cooling Fan - Retail
Air Flow: 53.24 CFM
Fan Dimensions: 120 x 120 x 25mm
Features: High efficiency system thermal solution Higher airflow and less noise
with 9-bladed design
Noise Level: 26.6dBA
As the fan is going to go in a side window, you could also look at a fan with
LED lights for a bit of flash.
You will probably have to use your own screws to attach the fan on the side
instead of the ones that might come with the fan, as the usual screws are
designed for mounting on metal. For an installation similar to yours, I went
to the hardware store and got some longer screws and nuts to mount mine.
If you need any clarification, please feel free to ask.
Search strategy: Personal experience.
Looking Forward, denco-ga - Google Answers Researcher