Go to http://www.rivesaudio.com they can give lots of good advice, and/or act as consultants.
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I agree with Narchy. Rives is a good place to start. He has a forum on acoustics too:
Rives Acoustics Forum
I helped a friend build one wall to enclose a listening room and he did not want to get too exotic. We just double layered the drywall, put studs on 12 inch centers instead of 16, and didn't spare any screws. It worked out quite well with little extra costs.
But ask at Rives' forum. He is a pro.
I would put in another reccomendation for Rives. If I would have known about their service about 2 years ago when I built my own room it would have been worth every penny of their fees to help me realise the best acoutics possible in my space. It is almost impossible for a novice to get things right without any help, or it takes tons of experimenting and work to get things right, or you can get lucky. Rives can work with what you have to work with and can most importantly give you the instruction to build the neccesary treatments that your room will require. This alone is worth their fee. If you are running a top notch (usually pretty expensive) system then it's a no brainer to get some pro help. The room is indeed the most important component in your system
If I was in your position, I would build a 2x4 stud wall with drywall on either side. In addition, on the listening room side, I would put a material called sound board underneath the drywall. It's made by a company called Celotex. A big name in the building industry. Almost sound proof, it does a great job at deading the room.