You can also use some high quality tin snips to cut out a hole as noted here:
http://www.silentpcreview.com/forums/viewtopic.php?t=23...
However, the problem with both my suggestion and COLGeeks suggestion is that it is not really safe to cut out the metal while you still have parts inside the computer. Little pieces of metal might fly into parts and end up shorting out components.
Another option would be to:
* Remove your case side-cover.
* Mark a spot on the side-cover near your CPU.
* Cut out a hole for a 120mm fan using either tin snips or COLGeek's suggestion.
* Attach a 120mm fan (using glue or screws or whatever) to help reduce the heat around the CPU.
* Also, you have to decide if blowing cooler air into the computer or pulling hot air out will give you better results in your setup.