Will new windows help my energy bill?

So I recently moved from one older home in Houston to another old home, but bigger. Energy costs at the old home (1900sqft) was manageable in the summer rarely going above 2000kwh. New home is 3300sqft with a pool and two AC units the energy usage projection is crazy at 4,000kwh, I already plan to install a variable speed pump for the pool which should make a difference. My question is, has anyone installed new double pane windows and noticed a significant energy usage reduction compared to having single pane? I know there will be a difference but was wondering how big of one others experienced. We keep the temperature at 76 in the house to keep it manageable, we do have very large windows and lots of them. Any advice/suggestions appreciated.

I also am interested in getting the insulation in the attic upgraded to R38 or whatever is the standard these days, anyone know what they typically costs?