Harold's suggestion of the trol-a-temp might be your answer. You didn't mention return ducts though. If your upstairs return is in the hallway and there aren't any returns inside the rooms upstairs then you might need to cut an inch off the bottom of the doors upstairs to let the air out of the rooms and into the hall. Just a thought... Also, before you go to all the expense of installing separate zones it would be worth having your hvac company see if they could balance the system a little better if you haven't already tried. I was all ready to go with the powered damper zones etc. and instead they managed to balance the ducts well enough in 2 or 3 visits to get it just fine. Every house is different though so you might need zones.