I added a 120mm exhaust fan to the top of my case just a few days ago. It was my first attempt and it turned out to be a fairly easy case mod.
A local Ace hardware store had a Makita 4 1/2 inch hole saw that will drill/cut metal and a mandrel (special drill bit that is attached to the hole saw). There are other brands available but Makita was the least expensive. It took about 20 seconds to drill/cut the hole using the hole saw, mandrel, and an electric drill. The 4 1/2 diameter hole is just a little smaller diameter than a 120mm fan. It turned out to be the perfect size. The edges of the hole were were fairly clean. I didn't have to do any filing or nibbling. I used flexible case molding around the edge of the hole to give it a nice clean look. I found a 120mm chrome fan grill at a local CompUSA store that worked with the molding. The grill was slightly raised so it cleared the molding. Most fan grill are flat.
As previously mentioned, empty out the case because metal shavings wind up all over the place. I had a brand new case so all I had to remove was the hard drive cage at the bottom front of the case and the stock fans that came with the case.
Before you drill/cut be sure to check the placement of the fan. I inserted my power supply in the top rear of the case and an optical drive in the top front of the case to make sure the fan would not be in the way. Remove both items before you start drilling/cutting.
Wear safety glasses and gloves. The metal filings are sharp and dangerous.
The mandrel has a drill bit that makes a pilot hole to get things started. It also keeps the hole saw from moving all over the place. When the hole saw makes contact with the case the case will shake, rattle, roll, and get damaged if not held firmly in place. I didn't have a vise or some other device so I placed the case against an inside corner of a wall and used my feet and knees to hold the case firmly in place. Keep a tight grip on the drill too or it may It's not an ideal solution but it worked. Use the fan grill to determine where to drill the holes for the fan screws.
The hardest part and most time consuming was cleaning up.
NOTE - The fan at the top of the case should be used as an exhaust fan blowing hot air out of the case.
GOOD LUCK!
Find a gun like the one in NewbieTechGodII's avatar, and shoot your case! 
That thing doesn't shoot holes in you, it blows you into pieces. Arms and legs in different directions. 
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