I saw this stuff called "sound board" at Lowes/HD. It comes in 4 x 8 x 1/2" sheets for less than $10. Is it any good? Forgot to mention that it's a sound insulator, but I guess most of you knew that anyway.
Two years ago when we built our house I had them put up a sheet of Owens Corning sound board to deaden sounds coming from the tv in the greatroom through to the master bedroom on the other side. It seems to work wonders unless the tv is really blaring. At normal levels nothing transfers through, though this is not where the HT is located so Y.M.M.V.
I bought a sheet to test it out and it seems to work pretty good on the mids and highs, not so well with the bass. I'm trying to decide if it's worth the trouble to line my walls with it and then put some sort of interior wall board on top of it.
I don't know that I would use it after the walls are up but if I ever do another HT, I will use it before the drywall goes up. I think it would be a good alternative to the "staggered stud" room design to isolate the room.
I think this sound board works mostly by soaking up the upper mids and highs. If you cover it with drywall, you ruin the effectiveness. What you could do is cover it with a loose open-weave fabric. A few panels in key locations would make a big difference (like the point where you get a reflection from your fronts to your main listening location. use the "slide a mirror along the wall until you see the speaker reflected in it" technique to find this region.)