There are generally two types of gas used with windows. Argon and Krypton. The purpose of the gas is to improve insulating properties between the panes in doubled paned windows. You want to inquire whether or not the windows will have gas.
Reflective glass coatings are most effective against radiant heat (direct sunlight).
Although there can be manufacturers that offer limited labor warranties with their products, most labor/installation warranties are provided by the individual contractor/dealer.
A U-value of 0.25 (R-4) is a good rating of energy efficiency.
When having new windows installed you should pay attention to the insulation and air-sealing around the new window frame at all points. The difference between what many manufacturers call “New Construction” windows is that they are typically stock sizes and have the integral nailing fin.
Windows are typically about 10% of your house, but are often responsible for about 40-50% of the heat lost or gained.
Low-E coatings are applied to the glass at the end of the float process, and they are generally soft-coat. Hard-coat Low-E is not as effective and thus rarely used.
Aluminum windows have fallen out of favor for residential use due to their conductivity. Their primary application is for commercial properties.
You can achieve 2-3 times the efficiency by installing new windows verses an old window with a storm.
Article courtesy of Tim at Replacement-windows.com http://www.replacement-windows.com/replacement-windows-tips.php