Just make sure you have low-e and argon and then you want to check the u-value, which is the recipprocal of r-value but the number your going to see on the labels of all new windows. Anything below .35 is good.
The R-value ratings given by most window manufacturers is center of glass only (about the size of a quarter). The U-factor rates the amount of energy that gets past the entire window and is much more accurate when determining energy efficiency. Low-E / Argon window technology is from 1982 and is archaic and .35 is a terrible rating. Try a company like Schuco, they have U-factors as low as .16.