While the type of lights is a factor, more important is to have the sceen lit in such a way as to keep shadows off and have it lit uniform from top to bottom. We recently shot on green screen and most difficult was lighting the bottom where it curves onto the floor. For us this meant top and side lighting upstage of the actors, and lighting the actors separately with less intensity than the screen was getting. All told, I think we had something like 14 fixtures being a combination of Arri 1ks, and halogen washes.
Even with all this light, to keep shadows off the screen, we had the actors at least 5 ft. downstage from the vertical part of the screen. We were shooting on a 30' wide screen with a vertical curve at the foor and it's amazing how small it is, and how limiting your camera angles are when the actor has to stay that far downstage. Given the chance to do it again, we would probably go with at least 40' wide. Understand that this was to support a completely virtual set, not just green sceening in an object.
We were shooting with the 60D.