U-value...MUST be .35 or less. Ignore R-value when dealing with windows. It is a meaningless figure when comparing window efficiency.
Solar Heat Gain Coefficient - SHGC - important in the south because it describes how much direct solar gain the window will allow to pass as heat. Important in the north for the same reason. Lower is better.
.35 or lower in the south...officially, no limit in the north. Not everyone agrees with that, however.
Absolutely ensure that the windows have a LowE coating. The reason a dual pane window is much more efficient that a single pane with storm is the coating. Without the LowE, a dual pane window isn't really much better than a (tight) single pane and storm.
If you want more specifics on any of those suggestions, feel free to ask!