If you are going to put it on top you may as well use the fridge to keep it cool, put computer in the fridge and overclock it.
* The advice given should not be taken seriously!
Yeah, the computer-in-a-fridge question has been asked numerous times on GD. Not sure 100% what the response is, but it's generally not advised as it's do with water buildup due to condensation which can wreck the PC internals.