I would put the poly under the plywood,on top of the insulation if you are certain that the moisture is from humidity condensing on the cooler concrete slab in the summer. Harold might have different ideas. Just in case, I would use pressure treated studs and the rigid pink or blue insulation. If you do this, it doesn't much matter if you have a little moisture in there, as there will be nothing to rot or be ruined. I would also be sure to glue and powder nail the 2x's down so you don't have a banging or sqeaking floor. Remember, this is the time to run any speaker wires, phone wires, network wires, cable tv wires or thermostat wires, under your new floor. Do more than you think necessary as you can't do it later.