Ok to actually ANSWER your question, unlike the first person... a hole saw is the best way to go about doing it. Or, you can use a hole drilling attachment on a power drill. You just go to Home Depot or your local hardware store and buy the appropriate sized cutter (80mm or whatever you need). What material is your case? This makes a difference when selecting the cutter. Never use a wood cutting hole saw on aluminum or steel, you will shred up the blade and I have seen pieces fly into people's faces before.
As for the screw holes, just match up a drill bit to the size screw you want to hold the fans in with, and drill away. I usually lightly pencil the case before drilling any holes.
Of course, I have to say this, be safe when doing this, wear protective glasses!
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