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.

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

I have even thought about moving into another house but I can't really afford a single family home in the D.C. area.
The best way would be to build another room inside the existing one and try not to let the ceiling and walls touch the originals. Using wire to hang the joists from the existing ceiling will minimize sound transfer. You can use 1 inch metal rails for the joists or even 2x4s. Adding insulation and having two doors between the rooms will help further. Using an exterer door with weather stripping is a must also. Bass will be the hardest to stop but the mids and highs can easily be reduced or eliminated.
One of the "objectives" you can get Rives to look at in a design is soundproofing. While that was not a concern to me, they were great to work with (you can find out about them from other forums or their website).

(PS-am a customer of theirs, not affiliated in any way)
The other option, assuming you don't rent, is to hang celotex sound isolation board on top of the drywall. Won't look that pretty, but you can hang drywall on top of that for excellent sound isolation. I also think that building another room inside your current one is a good idea albeit quite expensive and fairly time consuming.
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: