There are also a few other methods to prevent YLOD from what I've read. There are some power supply models in the newer Phat model PS3s that run much cooler than the launch models. It is possible to replace the power supply to reduce heat output.
Another method some suggest isn't to buy an external fan attachment, but to replace the internal fan with a 19' blade fan instead, as that takes power away from the PS3 and makes it overwork or something. I'm not completely sure if that's true or not.
This website:
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Has some pretty useful tips.