At bare minimum, a modern replacement window will have two panes of glass, and will often have argon gas fill between the panes as insulation. Windows in this class will carry a Limited Lifetime Warranty on vinyl frames and 20 years on hardware, screens, and glass.
Example: Simonton ProFinish
Windows in this class are highly recommended for their overall performance. Expect a window in this category to have double or even triple panes, argon gas insulation, and a Double Lifetime Warranty. Any window in this category will carry the Energy Star label and will qualify for home tax credit.
Example: Simonton Reflections 5500
Windows in this class are elite in every way. Double and even triple pane glass is available. Impact-rated hurricane windows for High-Velocity Hurricane Zones and Energy Star ratings for the harsh, hot, and stormy Southern zone. Sound-proofing by a 50% dampening over standard glass.
Example: Simonton Stormbreaker