We made a drum isolation platform out of 2 sheets of 4'x8' 12mm MDF with strips of Auralex Foam glued to the bottom to support it. In our old studio this did a fantastic job of isolating the drums from the floor. We had a coal cellar underneath the live room, and so any instrument or drum would vibrate all the other drums, and it just sounded pretty messy in that room. Once the drums were on the platform it really transformed the sound and made the drums sound a lot more focussed. We were expecting maybe a tiny difference at best, but it made a very noticeable difference that we were very pleased with.
Our new liveroom has a concrete floor, but we use the platform anyway. I'm not sure if it makes as much difference to the drum sound in this room..
It could stop some of the thumping sound from your feet stamping on pedals if the platform is resting on 2 inches of foam, but it definately won't stop the problem of sound leaking downstairs.
As Fran points out, once those walla and floor are excited there's not a great deal that you can do if your room and the room below are part of the same structure.