Insulation, Radiant and Vapor Barriers - Vapor Barriers and Insulation

I am just starting to finish my basement. It has poured walls with thick padding insulation 3/4 of the way down the exterior walls (to below the frost line). I have two questions: 1) Is there any need or advantage to stapling 6 mil plastic to the back of the wall studs as I frame the walls? and 2) Is there any need to insulate the remainder of the exterior walls not already covered?

If you choose to put in a vapor barrier, it should be placed on the inner most edge of the studs, not behind them. Generally the foam/padded insulation has its own vapor barrier made in as a part of it. Also, basements are notoriously insulated by themselves, as they are mostly underground. After you install the stud walls, you could install R13 kraft faced insulation if you wanted to, as it couldn't hurt. The kraft facing is a vapor barrier and should be installed to the inside of the room.

Your climate will determine whether you need full insualtion or not.
How cold are you winters? You currently have 4 feet of insualtion which tells me you live in a cold climate. It would be ideal to extend it to the floor if you want the basement to be a liveable, comfortable environment.

Some Codes require a "moisture barrier" to be placed up against the concrete wall (starting at grade and extends to the floor). This prevents contact between the insulation and concrete.