waste of money mate. It is hard to apply on some places on the case (eg, behind the mboard) most noises come from the fans and the HDD as well as your DVD/CD drive. But if you wish to experiment i will give you a recommendation to do it without any cost.
Go to a fruit market and ask them for those foam pads that are used to put under fruits so that they dont get bruised while being transported. It is the same material and fruit vendors usually throw these out. Get enough for your experiment and come home and try to apply on your case (using glues, sticky tapes, blu-tac or anything you desire)
Note. If you are not careful you can disrupt the air flow by blocking those air vent holes on the case. Dont put them around HDD or CDdrives because it will increase the temperatures in those drives due to zero air flow.
Goodluck and have fun
