I'll second the physical danger of working on a PS, but I see a second issue as well. If you install a fan that doesn't provide as much airflow as the original, you will likely drastically shorten the life of the PS and will have to replace it anyway.
There are sites dedicated to "silent" (really low-noise) PCs, like http://www.silentpcreview.com/
. Take a look at their recommended list of quiet power supplies.