This is a common problem in split-level houses, especially those heated and cooled with warm air. Warm air heating/cooling systems are always a compromise because heating is done best from the bottom and cooling is done best from the top. Central air tries to do both, sometimes unsuccessfully. Adding separate air conditioning ducting from the attic with a separate thermostat can solve the cooling problem more easily than trying to retrofit zoning controls to the existing ductwork. Moving the upstairs thermostat downstairs could help after that is done.
For hot water heating with separate air conditioning you can visit http://www.hydronic.net