Low-E windows are the way to go if you're building a home and can afford the asset. Even if your remodeling your existing home Low-E is still the way to go.

What is a Low-E window? Well a Low-E window is a double paned specialty glass window filled with usually with argon as an insulating gas.

What does Argon do for you:

Cooler in Summer - The total solar energy transmitted through Low-E windows is almost 50% less than standard insulating windows.

Warmer in Winter - The insulating value reduces heat loss and heating costs in cold winter climates.

Reduces Ultraviolet Energy - Low-E windows help protect interior furnishings, fabrics and carpets from ultraviolet damage.

Transmits Visible Light - About 88% of desirable visible light transmitted through this glass allows the exterior appearance to be similar to clear glass while providing glare control in bright, sunny climates.

Oh yea Argon - Argon is nontoxic, clear, and odorless gas that provides extra insulation. Since argon is heavier than air rather than being in continuous motion like air, it greatly reduces the transference of heat and cold through glass window panes.

Source: http://www.reeddoorworks.com/windows/low%20e.htm