Matt, 1. Higher STC will reduce the noise that is coming through the window assemblies. If that is the dominant source of noise, the difference will be significant and especially with something in the 40+ range like the Milgard product. Depending on your wall construction, your walls will be the weak point in the assembly as will the ceiling/attic assembly. 2. They do work and are well constructed. They come as a pretty penny so be prepared for that number as well. When you are talking about the Milgard windows, just go for the top end. You are already spending a good chunk of money on the window to begin with and the install. The delta between the three levels of STC is not that significant if I recall correctly. Good luck.

Thanks to SJ2012 for your honest and specific comments, which were exactly what I wanted to hear (for some reason, I was never notified that anyone had responded to this thread, which is why I haven't been back earlier). Someone suggested just replacing the windows with new laminated glass ones. The thing about that is, with 2 sets of windows, you are getting a double barrier, and I know that Soundproof Windows' STC ratings are based on having the window OVER another window. The sound reduction will be markedly less without the extra windows, as a simple double-pane window has an STC of 26. With the Soundproof Window added, your STC rises to 36. That said, I am very concerned about mold problems. My windows get tons of condesntation on them in the winter and spring, and the result is mold, not to mention the screws of these supposed lifetime windows rotting out (the company that made them is of course out of business). If I have 2 sets of windows, I am thinking that the condensation will form on my original window, and I am going to have a heckuva time getting rid of it. Which leads to anothe question--with double-hung windows, how do you actually open both? I would very much like to hear more from EcoStarRemodel about the thicknesses and about installation. I am also curious about the impact of different materials for the frame. GulfBreezeWindows, can you also say a bit more about what sort of windows I should look for? I have double-pane windows. Yes, they reduce sound. But when you live next to a busy highway, you need a lot more! I have experimented with closing the windows to my truck, then opening them. The sound reduction is phenomenal, and far beyond any house window I have encountered. The reason--the lamination!

I agree with your assessment millworkman. To the extent that I can feasibly soundproof walls without having to tear into the wall too much, I would consider that in addition. Given that I'm on the 5th floor of a condo building (i.e. its not just my preferences that matter), I may have problems doing much. I plan to get a professional assessment as to whether there are other avenues for noise pollution to get into the unit. Again, would have thought that it would be fairly easy to have a number of options for replacement soundproof windows in the NYC. If I had any handyman skills or interests, it'd be a good business opportunity.

Talked to a local window company today about their windows for sound control, which are essentially a double pane window with laminated glass. They are a distributor for the following windows: http://www.vinylbilt.com/lib/pdfs/SSP_Glass.pdf The salesperson told me that upgrading my exiting windows (double pane windows by Vinylbilt, about 4 years old) with these ones will have marginal effect and will reduce the noise by about 10-15% based on his experience. Not sure how much I can trust this. I was determined to go this path but now have second thoughts. Anyone has experience with double pane laminated windows for sound proofing? I have started reading about soundproofing recently with no prior background. I feel that I know a lot more than every local window company that I have talked to so far, and these guys claim to have been in the business forever!