There is a lot of good information on the site. I found a document on a site called Efficient windows.org.
Here is what it said for Oklahoma.

South/Central Zone U-factor
Windows: U��0.35
Skylights: U��0.57
A low U-factor is useful during cold days when heating is needed and is also helpful during hot days when
it is important to keep the heat out.

South/Central Zone SHGC
Windows: SHGC��0.30
Skylights: SHGC��0.30
Windows with low SHGC values help reduce summer cooling demand, but they also reduce free winter solar
heat gain.

So if I choose a window with U factors LESS than 0.35 then I should have a good window? (depending upon a good installation job.)

If it choose a window with U factor of 0.25 and an SHGC of 0.22, that is even better?

Thanks for your input. It will not let me post the PDF doc, but you should be able to find it.

Here is a link that might be useful: efficient windows standards, online tool