The proper procedure to have a comfortable and healthy home and save energy is to have a whole house approach. Before you do anything with your house, do a home performance assessment. The report of the assessment will show you how and why you experience uneven temperature in your home, and what steps to take to make your home more comfortable and health and save energy. Air sealing than add insulation is the most efficient to save energy and have more even temperature in a home. It can help you save you 15 - 40% on your utility bill, depends on how leaky your house is. After that, you may only need a smaller heating and cooling system.
A HERS (Home Energy Rating System) rater or BPI (Building Performance Institute) certified contractors can do the job for you. If you live in California and want to get the Energy Upgrade Rebate, only the BPI certified contractors are qualified.