Qacer,
90% of the materials that will help to deaden sound will also act as an insulator. Try one of the above materials, and if your heat starts to rise, consider adding a case fan to help move more air through your case. Yes I know that another fan adding it's share of noise is kinda defeating the whole purpose of adding sound deadning material, but you don't need a "killer" fan, just something to help move more air. Those slot coolers are really quiet and might be just enough to help take care of any extra heat you may experience.