The CPU is going to get hot, really hot and so you need to keep it cool. This was covered above under the topic of CPU Cooler. Yet that is not everything as the GPU will also generate heat and both the CPU and GPU will be inside a metal box.
All cases will have a case fan or two but quite a few of the larger cases have a few prepared locations for an extra fan or two (or five).
The question of how many to add depends on how you plan on venting the hot air. There essentially are three different situations in the case.
- Positive pressure
- Negative pressure
Neutral pressure simply means that the amount of air coming in is equal to the amount going out. Positive pressure is when more air comes in compared to the amount going out and conversely the negative pressure is when more air is coming out compared to coming in.
It is impossible to be truly positive or negative pressure, in that situation the extra pressure will equalize via any crack or opening in the case.
Obviously the best case would be neutral pressure but failing that positive pressure would be the next best. The over pressure will escape via cracks. This usually means that the incoming air is filtered and then escapes but the negative pressure situation will have unfiltered air entering into the case via all cracks or openings. The unfiltered air means that dirt will begin to accumulate in your case.
There are different sizes of fans but the most common sizes are 120mm and 140mm. Larger fans will push more air which means that they can spin slower and will be quieter.
Air pressure examples