its called "de-coupling" your fans. basically instead of the metal of the case vibrating against the plastic of your fan housing, you put a layer of rubber in between to dampen and reduce the noise caused by vibration.
Its something you should look into doing after youve replaced: psu,case,fans, and suspended hd. the difference it makes it very minimal after that but just for the people like me that are anal about it, it help=P
but really, silentpcreview will answer ALL the questions you have about silencing your rig, browse the forum for about 30 mins and youll learn a lot.