Wow. I wish I would have known about this sooner.
Here is the basic scoop on a few things.
Good (high R value) "pink fluffy" insulation will work WONDERS for what you are trying to do. No need to go Roxul if you don't want. The type of insulation is NOT your weak links.
Any air ducts are link perfect communication conduits for sound from one room to the next. If you can even replace some of it with 8 to 10 feet of flexible insulated duct (not too pricey) and put a few bends into it, the sound will get absorbed a lot and not pass from the theater room to the rest of the house. I thing that I used two 15 foot pieces (one for each duct) and I could put one end up to my ear and have someone yell in the other end and not hear it... Seriously.
The other major issue is that you don't have enough "stuff" to stop the sound. For the walls, the double drywall (5/8" if possible) is your "mass." More mass means it takes more energy (sound waves) to move it, thus it dampens the sound. That is also for your ceiling.
Add the Green Glue (heavy on the ceiling if you are doing DD up there) and it adds that visco-elactic element that literly dampens even more vibration and creates a barrier between the two sheets of drywall to reduce sound transmission from one sheet to the one behind it.
Which gets me to decoupling. This is where some people will go staggered stud wall like I did, or a double wall "room within a room". This really holds the sound in (and out) by not giving it a solid path to go from inside the room, through the drywall, through the stud, through the outside drywall, and into the air outside the room. For me, I planned on just loading up a lot with Green Glue on the ceiling as our master bedroom is right above the theater, but I quickly learned that I would not be happy with that. So I ended up with Whisper Clips from SoundProofingCompany.com (where I also got my Green Glue at an "AVS Member" discount price), and bought the hat channel locally for less than I thought, and I decoupled my ceiling as well. I was really amazed at how much sound is now trapped in the theater. Sure, some LFE stuff gets out, but lets be real. It takes something just short of a nuclear bunker to hold super low frequencies back. I can, however, easily watch a sci-fi flick with all sorts of explosions and special effects craziness at a pretty good volume (louder than my wife likes in in the theater) and she can be in the bedroom and not wake up. To me, that is a success. I can't image what it would be like if I hadn't spent the extra $100 or so to decouple the most important "wall" in my theater... the ceiling...
I know you are at the 11th hour on getting moving on this, but just think about it before jumping one way or another and for a little extra time and a little extra effort (clips and channels are actually pretty easy to work with) you can know that you took it to "the next level" without going stupid crazy/expensive.
If not, hey it is your call. I can only offer advice. You must choose what fits for your needs/wants/dollars.
