The graph below shows which structural air-leaks drive up energy costs the most, allowing you to maximize efforts toward reinforcing your home's building envelope.
You can chase smaller oppertunities all day long to tighten the home but if you miss the big oppertunities like in your attic you will never have a tight, well sealed home.
The attic usually has about 40% of the leakage in your home. This includes top plates on that attic floor that leak air into or out of stud cavities that are connected to holes like light switches and outlet, can lights (usually a huge source of air leakage) and other penterations like ceiling fans.
When ever you improve the "weatherization" of your home but miss sealing the attic leakage, you're doing little to actually improve the home. Always go for the biggest bang for your buck and in this sistuation its the attic floor. This means that before you add insulation you should think about removing the exsisting insulation and sealinging the attic floor before you add the insulation you want.
Never make this decision without using a blower door test to prove its effectivness for your home. Good luck with making your home more comfortable.