The answer depends on your goals and the age of your house and windows.
Please post back with more details about these items and I will respond again.However, please see below for some general considerations.
Building science principles often show window upgrades are a final step in improving comfort, health, and/or efficiency in homes. This is because of three main things:
- Windows are a relatively small % of the surface area of your home;
- The difference between high performance windows and normal windows isn't as significant as insulating attics/crawls/walls (and insulating is way cheaper); and
- You want some degree of air leakage in homes (to exchange stale air, potential pollutants, moisture, etc.) and windows are a great location for air leakage because they connect directly to outside.
Unless you install an energy recovery ventilator (or other whole house ventilation), then after properly sealing your attic/crawl, you will need leaky windows to maintain sufficient air exchange.
For more information:
Read "Is there a way I can improve the sealing around my windows to increase their efficiency?" a Q&A answered by Danny Kelly.