You may want to start by getting quiter fans and coolers. Sound dampening material is good for dealing with sound from fans and what not but isn't very good for vibration sounds. There are rubber gaskets availible to use when mounting fans and drives that help to prevent vibration sound. Some of this matting will trap heat reducing the cooling efficancy of the case. Not to mention the cost involved.
I agree and would add that a quieter PSU might also be effective. XOXIDE, among others, has a good selection of silencer kits. 8)