$600 for replacement windows? Installed or material only? You are paying too much, even if it is triple glazed with Krypton and double strength glass.
Simonton Prism Platinum or Simonton Reflections 6500 (same thing different name). Double paned windows, glass breakage warranty, Argon insulation, Low e, stainless steel balances, AAMA gold seal... very nice window. Even if you pay full price, for material, it's not going to be $600 for the average sized window (85 UI). With my discount, I'd pay something like $180 for this window.
Climate Guard makes a slightly less "fancy" window with all the same basic features, for less $$$. I have Climate Guard windows in my own home. Call Linda at Remodelers Supply at 847-985-1700. I have to call her at 8:30 to place a window order.