Do you know if the main attic is insulated - if not it should be, as should your walls. I don't think a plastic sheet will do much otherwise.

The most important thing is to make sure the floor of the main attic has insulation up to (I think) R30, the new accepted standard. Before you consider putting up a barrier in the attic, make sure that you do not impede air flow, which could introduce new problems.

A properly insulated ceiling (ie the insulation is on top of your ceiling drywall when you're looking in the attic) has no need for what you're proposing, and I agree that a barrier is a bad idea. Plastic would not be the right material anyway.

The roof and/or soffits should be ventilated, believe it or not. You want the roof space to be as cold as possible/close as possible to the outside temperature... counter-intuitive as it may seem, you want this situation because you want to reduce the instance of ice damming on your roof, where snow/ice melts and refreezes continually, because the ice can force itself under your asphalt or other roofing, causing potential leaks.

Apart from a decent amount of insulation, you want ventilation, ideally something active like a whirlybird.

I myself plan to add additional insulation to my roof space (blown in) once I have the recessed lighting I want to install in place...

R50 is what they are now pushing as the acceptacle standard for attics according to CMHC flyers at my local Building supply . Who has that?

Yes, you want your attic as well ventilated as possible to prevent moisture buildup leading to damp insulation and heat buildup leading to ice dams on eaves. Make sure you have the proper amount of attic insulation. Also, don't neglect doors and windows where a LOT of heat can escape - the site listed below provides some good tips in that regard.

Here is a link that might be useful: Insulation and Weatherstripping