Energy Boomer has a great article about what to look for when buying windows.

Here in Arizona, you should pay the most attention to the window’s Solar Heat Gain Coefficient (SHGC) rating. The SHGC is the fraction of incident solar radiation admitted through a window. SHGC is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits.

Click Here to read the article.