There are companys that make retro refit windows to fit the old style sashes like you have. They look like the old style windows but have modern R values. Saw a episode on "This Old House" about them.
But saying that, compleat new windows offer a far greater R value. You do away with the sash pockets (open void) and have a better seal/fit in the openings at a lower replacement cost.
It's like driving a 1927 car across the country......yes you can do it but it's a much nicer trip in a 2010 model.
E coated, argon filled, tripple glazed beats single diamond glass anytime.
