Can I soundproof a rec room that's already built?


I live in a three story townhouse. I want to listen to my stereo in the recreation room in my basement at relatively high volumes from time to time (also home theater). The place already has drywall ceilings and walls.

Am I kidding myself that I could make a big dent in soundproofing even though the walls are already up? Would it just be a waste of money? I don't want to just cut down on some of the sound from going upstairs and into the units on either side of me, I want to cut down on a LOT of the sound.

p.s.
I am not willing to tear down my walls and ceilings.

p.p.s.
I have even thought about moving into another house but I can't really afford a single family home in the D.C. area.
mrvegas
How do you "hang celotex sound isolation board on top of the drywall?"

Don't you have to mount channels or something on top of the drywall before you hang the celotex to create air space? How would it work on the ceiling?

THanks for all the input folks.
Go to our resource page. Look in the column of acoustical articles. You will see the 10th one on Sound Isolation. I recommend you read this. It will give you some valuable insight I hope.
Yes, you could put channels on top of the drywall, and then add celotex, this is the prefered method, as you will have an air gap. Locate the studs through the drywall and screw the celotex to the studs. Then hang drywall on top of the celotex. Ceilings work the same way. Locate the floor joists above and install the same as the walls.
On a soundproofing scale of 1 to 10, if 10 represents the soundproofing that could be obtained by tearing down the existing walls and rebuilding staggered walls (etc etc), and 1 represents stapling egg crates to the back wall, what do you guys think the scale would be for putting channels on top of the existing walls and ceiling and then adding a layer of celotex and drywall?
Look at the following site, they have a lot of information about soundproofing and acoustic treatment:

http://www.soundproofing.org/index.html