Get a normal dremel with a disc cutting wheel. Reciprocating saw is less suitable for precision work and you'll have to do more edge sanding to achieve the result that you want.
Ditto! Abso-tively, Posi-lutely! You can get your basic Dremel kit at HD for a little over $50 as I recall. Either HD or Harbor Freight sells a similar tool for less than $20. You'd want to get a package of the abrasive cutting wheels -- they wear down with use. There is a metal "saw" wheel for use only with plastics or wood, but that also can come in handy for a case mod.
It takes time, patience and experience to cut a 120mm fan-hole with the Dremel, but I'd think the job might look a bit neater than with anything like a reciprocating saw. Some mod-veterans recommend 3" and 4" diameter hole saws used with a drill for this purpose. It's also nice to keep an assortment of files handy to eliminate sharp or rough edges.
There's also another interesting little tool available for about $9 or $10 that looks like an office hole punch or pair of pliers -- the so-called "nibbler." Very useful.
DO NOT MOD YOUR CASE WITHOUT FIRST REMOVING ELECTRONIC COMPONENTS! And make sure you clean out the filings thoroughly before re-installing your parts. If you can -- blast it with a garden hose.
It IS possible to drill a hole
here or there without this precaution, but I've done it by covering the components in plastic drop-cloth and covering the exit-hole area with tape so the tape isn't penetrated by the drill-bit. The inside of the tape should provide a sticky surface to catch filings, or squeezing the tape flat as you remove it should capture any tailings left by the drill. This is one of those "afterthought" short-cuts I've used in a hurry, and I don't recommend it.
