Fans and case modifications
These days every computer has at least two fans, with many overclocked systems having plenty more than that. The question then arises, where do you put all those fans? Do you aim 64 high powered fans in the shape of a dome around that speedy little CPU like so much TNT around a plutonium pit, or do you stack them one atop another in hopes of creating some 12V powered subsonic wind tower? Well the answer my friends is blowing in the wind, so to speak.
The point of all these fans is not to create a tornado of wind within your computer but to bring cool air into the case, speed it over the heat sink and then exhaust it from the case.
Inlet fans, where do you put them? This question depends on how committed you are to keeping things cool. For those that treat their cases like an expensive car, put the inlet fan in the front of the case. Most MFG. provide what looks like a speaker mount (and it may be, but why use that cheap little speaker) there. Modifications will depend on the size of fan you are using but will probably be nothing more than drilling a hole or two in the frame behind the face plate, which will remain hidden behind the plastic face plate.
With the inlet done, lets take a look at an outlet. This should be in the back of the computer and if possible behind the CPU. Chances are there will not be a place for those unwilling to cut their cases up to place a fan. If such is the case, remove the plate where all the jacks from your motherboard poke through (cases now-a-days generally come with two or three of these plates as jack configuration depends on your motherboard layout). This hole leaves enough of a space that air attempting to exit your case will naturally flow through. Air, like humans, is lazy and will take the path of least resistance generally. We'll deal with CPU's in just a second.