You might call the local Building Inspector's office. They often are aware of common construction practices in different neighborhoods.
You said you were finishing out this room. To be a theater? Is that the kind of sound we're talking here?
EDIT: Just saw your thread.
I'd consider treating all 4 walls and ceiling.
I'd either apply clips and channels to that wall or build a new wall in front (preferable). Double 5/8" drywall or comparable mass everywhere.
Before you start framing anything, it would be good to determine your HVAC plan. Also the door plan. Both of these are big holes, and after all, life already has enough big holes.
The Soundproofing Company