Soundproofing "after the fact" (after the room is already built) is really difficult. You are going to do more for sound quality in the room than anything else with what you propose.
However, decoupling the subwoofer might help with some of the sound going downstairs, but it won't do really anything for keeping those long wavelengths in the room.
Some cheapy DIY options are to take a piece of MDF (or even 3/4" plywood), and attach it to some thick foam pieces on two sides. This creates sort of an upside-down 'U' with the two foam pieces and the wood.
That will help reduce some of the transfer down below, but not much since it is still long soundwaves. It will, however, help with your bass quality in the room to some degree. This is the same thing that the Subdude is, only doesn't cost $60 plus shipping.
