I have used a hole saw bit for my drill. It works wonderfully. The 3" hole saw for use with a drill will cut a perfect hole for a 80mm case fan. Be sure to get the METAL cutting hole saw because the wood one will NOT work. Believe me, I have tried.
It looks like this except it must be one that cuts metal (including the arbor, it should be $25-$30) Be sure to get one that matches the chuck size of your drill. The 3" saw usually has an arbor with the larger chuck size:
http://www.internationaltool.com/milwholesawkits.htm
Another good tool is a Dremel. It is like a hobbyist's super tool.
http://www.dremel.com/html/home_fr.html
You will also need a small normal drill bit to make holes for mounting the fan. This seems to be the hardest part because there is little room for error and drill bits like to walk on sheet metal. You
can use a hammer and nail for this as well, and it may be more accurate, but I have never tried this method.
Dave