This topic is a little controversial. I'm one of those who believe the plastic belongs against the concrete wall, versus just behind the wallboard. Conventional vapor barrier installation defines the "warm side" of the room as the proper location. But, in a basement, the vapor is most likely to emerge from the concrete wall (which could wet/impair the insulation). Some of the traditionalists will claim that having the plastic against the concrete will trap water and cause a moldy smell - something I haven't experienced. My vote - put the plastic against the concrete wall. If you haven't purchased the insulation, then consider R-13. It fits into the same size cavity, but for slightly more money you get a 20% better insulating value.