I went ahead and grabbed a piece of fuzzy carpet remnant from a friend who just built a house. I put a small piece under the power supply near the CPU, and a large piece that completely covers the side of the case above the CPU. (I did wash it and brush it to get as much fuzz as possible off - yes it looked silly but satisfied me I wouldn't mess anything up with stray fibers). This little mod made a huge difference on the amount of noise coming out of the case

I too did this once. Except I bought some specially designed Pax carpets to quiet the case. And I must admit : it helped. The only downside was : the temp in my case got A LOT higher from then on. I realised then that the carpet kept the case from loosing at lot of it's heat through natural convection.

It's because of this that I would not recommed such acoustic mats. To get your system quiet, it's better to invest in quiet fans and HD enclosures IMHO.