PC Cooling
For keeping your computer running in peak performance, PC cooling is definitely important. But if you're not doing any serious gaming, video-editting, over-clocking, or running any highly-trafficked databases (instead of on a server where they should be...) you shouldn't be overly concerned with excessive after-market cooling. Excessive heat gets generated from within the system from vast amounts of processing happening on the video card and/or the CPU, as well as from hard drives spinning constantly accessing, reading, and writing data. This type of activity does not occur from excessive email or internet use.
As far as cooling goes, it's important to start with a case that supports a good airflow. Many cheap cases are not designed with this in mind and so air can get restricted in various places inside the case, allowing warm air to build up in pockets until hardware components eventually start overheating and failing.
The main thing to look for in a case is that there is a decent amount of air that can come in one direction, and flow through the case where it gets exhausted on the opposite end. Many people make the mistake of throwing PC fans just anywhere in the case where a fan can be installed. DO NOT do this unless you are sure that it contributes correctly to the flow of air in your case.
Most cases with any sort of decent airflow have a fan or two at the front of the case, typically in front of or behind the hard drive(s), that pull air into the case, and in the rear of the case underneath where the power supply is typically located, is where there is a large fan blowing air out of the case. Side panel fans are also common and pull air into the case and directly onto the CPU fan so it has a constant steady stream of cool air to feed to the processor.
Make sure however you decide to set up your case fans that they are blowing air in the correct direction to keep air moving in and out of your case (remember to keep your cabling cleaned up as well).
Use Case Fans Effectively!
Case fans are very effective at keeping your system cool when configured correctly as explained above to keep an optimum air-flow in your case. You can also buy aftermarket CPU fans that tend to keep your CPU a little cooler than the stock fans that come bundled with it. There are several different styles, and some of them are pretty large and won't fit in the smaller cases, so make sure you buy one that fits in your case.
You can also purchase after-market fans for your video-card as well to help keep it cool, although unless you plan on overclocking, you probably won't need to go overboard on either aftermarket CPU or Video Card fans, but a little extra cooling can offer performance increases, depending on how much stress you're putting on your system.
