[Gravity]

In addition go to your local Home Despot, and instead of building a wall, get some 5/8" Fire rated (we call it X rated board in Canada) drywall, and some resiliant channel (i forget who makes it but don't buy it from an 'accoustics' company like auralex. I think the stuff is called RC-6 but most home depots carry it. it's a steel strip that isolates the drywall from the wall both physically and by leaving an air gap.

This stuff works well for floating a wall and may be a better choice then building a whole other wall. You should also pick up some PL brand Acoustiseal. It's a non-drying sealant that comes in a tube like caulking, and you can use it to seal any air gaps around the room. It helps to decouple as well as seal airtight any gap anywhere. If you DO have to build another wall, I'd use the RC-6 channel on the new wall as well as using Low-E foam (for larger gaps over 3/8"), and the Acoustiseal (for gaps under 3/8") to seal the wall off completely on all sides. Pack the wall with OC703 or Roxul Mineral wool and you should be good (the resiliant channel gives an air gap that is needed for the mineral wool to work effectively)

I did this on my studio build and got better then 40dB of isolation with drywall on only one side of the walls... if you put it up on two sides you should be in even better shape.