Well, I am about to do my own case mod, which does include a few fans... I have planned out my procedure for doing the cutting... first of all, draw the screw holes in pencil, then drill these... make sure you get it where you want! (measure it out so it is squared with the edges of the case)... then attach the fan (loosely) and draw out the size of the circle to cut using the edge of the fan case (the ciclre in which the fan rotates), then remove fan... cut circle... smooth off, then line with something to give a smooth clean edge (like what you would use for the window, if you add one, but smaller), then attach the fan and grill, and there you are! Preparation is the key! As I was told so many times by my ex-army Painting and Decorating trainer...
If you are using a 120mm fan, use a CD as a template. Works perfectly. As Hotsauce mentioned, tape around the template if you are using a jigsaw. With a Dremel or nibbler you don't have to be as fussy. If you have any available and are using a jigsaw, put a thin peice of plywood behind your cut, this helps reduce vibration and errors while cutting. If you can afford it, use a metal-cutting holesaw, as the cut is extremely neat.
I used a drill with googles. Start with wide masking tape to mark. Do the big holes first then match with actual fan. The small holes (mounting) are the hardest because they almost have to be perfectly aligned. If you have a grill and want a big "blowhole" use metal snippers.
Of couse be sure not to leave any metal scraps or the mobo will shorten out.
Measure twice, cut once. Use the interior side of the case/panel to draw lines on and cut from the inside side, too. Use ink for drawing lines and circles. The "lead" in pencils is electrically conductive.
All you have to have is: 2.5 inch or bigger clamps(and 4 ~1" x 1" thin wooden blocks to prevent the clamps from denting the sheet metal), a small common(flat blade)screwdriver["chisel"], a sharp screw or nail that's 1.5 inches long["center punch"], a hard-heeled shoe["hammer"], a 3/8" power drill(NOT cordless), 1/8" drill bits, a flat ~2' x 2' piece of wood to clamp the case cover to-- so that it's safe for the drill bit to drill into it for ~1/4 of an inch, and a full round needle file(for removing burrs from the fan's screw holes--plus for making the screw holes bigger. 1/8" is too small). And a half-round file for metal. A half-round needle file will do, but a normal size half-round makes it much easier.
Drilling ~150 1/8th inch holes around a circle for a 120mm fan is a royal PITA. If you gotta use basic tools for good cooling, you will mount those fans the hard way. But you'll HAVE those fans!
Wow! that row of fans looks great! You convinced me, I was debating on what to use when I cut my holes. I was under the impression that hole saws didn't come that big. Looks like I'll be making a trip to the local hardware store.