Bob - wouldn't using extruded polystyrene on the wall, then putting plastic between the drywall and the fiberglass create a double vapor barrier? XPS is a vapor barrier. If you put it against the wall, then put another vapor barrier under the drywall, moisture will be trapped in the wall the same as if you put plastic on both sides of the fiberglass.
Everything I've read and heard suggests to use the XPS on the wall, making sure to seal it up completely air-tight with tyvek tape and spray foam, then use un-faced fiberglass insulation between the studs, and apply the drywall to the studs with no barrier of any sort between the drywall and the fiberglass.
To pass along a link that was provided to me in another thread: