Most PC cases have at least a couple places to install a case fan. Many newer cases have even more than two: often two on the back, one in front and one on the side. Some even have one on the top (although personally I find these rather ugly). But, many such PCs do not use any or all of their available case fan locations. This is fine in most cases, especially if your PC is running cool and stable (why fix something that is not broken). But, if you are installing quite a bit of hardware and really need the air circulation, you might want to look at installing some additional case fans.
Generally, the idea of a case fan is to (1) aid air circulation into and out of the case, (2) provide direct cooling onto the hardware in your PC. To provide air flow, you want air to come in from one place and exit from someplace else. So, the normal placement is to install a case fan in the front location to suck air in from the front. Then, install another case fan in the rear of the case to blow it out again. The rear case fan will also serve to suck hot air away from the motherboard and CPU and blow it out the back. The power supply fan(s) will assist in doing this as well. If you have a second case fan spot in the back, you can install another fan to assist in removing even more heat from the PC. In many cases there is a side-mounted fan option. This fan is usually installed in such a location that the fan will blow right onto the expansion cards. So, if you have cards which generate a lot of heat (i.e. video cards), then you can install a case fan on the side to assist in blowing away that excess heat from a location that is not as easily reached by the rear-mounted fans.
With that, I bid thee happy cooling!
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