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As for palstic - only needed if you have a miosture problem. Tape a 3'x3' plastic to the floor. Leave it for a few days. If there is moisture between the floor and plastic, then you have a moisture problem. Otherwise, plastic isn't needed.
The way you've described should work ok. Put the poly vb down. Once you've insulated the floor, you've effectively isolated the underlaying concrete from to heat, thereby making it cooler and increasing the likelyhood of condensation (Putting carpeting directly over concrete can have a similar effect, especially in areas with a colder ground temperature). The poly will 1/help to minimize moister getting to the concrete and 2/ keep any moisture from effecting your floor.
I wood suggest using treated 2 X 4's and you may just as well use 1 1/2 foam, glueing everything down to eliminate squeeks (filling the space also helps to eliminate air which carries the vapor to the concrete).