Okay, now that I've done some more research, I may as well weigh in with what we're thinking of doing. Take down the existing drywall. Add a layer of insulation. (What works best?) Use Isomax and then install a layer of QuietRock. Would using one layer of QuietRock obviate the need for two layers of drywall? Or adding a layer of drywall to the subflooring?
We're dealing with ambient noise--talking, TV, etc--traveling both up and down and footfalls traveling down. We don't expect to eliminate sound completely, we just want to be able to move around upstairs without disturbing someone downstairs, and not be able to hear someone downstairs on the phone. Which leads me to the question, is this overkill? Should we just add another layer of drywall to the ceiling? Or blow in insulation?