I just recently came across someone that told me adding additional case fans is actually a bad thing. His argument is that companies that build cases know how many to put in based on optimal air flow. He also states that adding additional fans only creates more heat. Anyone agree or disagree with this?
Adding more fans to your case can be beneficial to cooling, as long as it doesn't impact air flow. You want a positive air flow which results in enough exhaust to keep the fresh air flowing through the system. If the fans create a negative air flow, you have more air coming in than out, so the hot air blows around in the case.
This is why I say agree & disagree, as the effects on air flow can have a positive or negative effect. If done right / tested right, the additional air flow will improve case cooling.
Agreed with tecmo34. If you plan your ventilation well enough, you can add 20 fans to any case. I don't know how you will create more heat with fans, but I know that you can stop the heat from ventilating properly, which can be a problem.
I added a fan to my case when I bought it two weeks ago (Antec DF-30) and the extra side fan increased airflow dramatically. It almost doubled the air that exits from the exhaust fans at low speed, simply because more air gets pumped into the case. This in turn lowered my idle temps on my CPU (with a Cooler Master Hyper 212+, fitted with an extra 120mm fan) another 1'C~2'C on average. It also lowered my GFX temps and CPU temps at load by about 5'C each. The extra side fan blows directly onto my GFX card, the MSI GeForce GTX460 Cyclone. The GFX card has no exhaust funtion, so it just blows the hot air from the heatsink around and around, so I added the side fan to take care of that issue, and it worked wonder.
Some manufacturers sell cases for a lower price with less fans, and then they recommend you fans to add to the case should you wish to. I know Cooler Master (amongst others) likes to do this, and their fans are sold with the line "Recommended replacement for Hyper 212+, CM 690, CM 690 II, etc." on the fan leaflet.
Plan stupid, and you will get crappy ventilation. Do it right though, and you will get the best cooling case you can with a few extra fans.