"We can't find the STC on these windows."

Without an STC there is no real way to tell.

Even with an STC, you may find there are other 'sneak' paths for the noise.

http://nfrc.org/label.aspx

if the company doesn't have the nfrc
label, keep shopping.
also do a site search @ nfrc.org for
noise reduction.

do invest in Low e. in cooling climates the location
of the low e coating will reflect heat out of
house, in heating climates low e is on a different
glass surface to reflect heat back into the house.

I've found that double paned windows
do lower outside noise transfer to inside
house. To what extent..I'd be guessing.
but my clients do comment on the lessened
traffic sounds.

as brickeye noted there are other paths
that allow noise to transfer. finding these
areas may entail having a blower door test to
locate these areas.

we used to have a window guy here named Oberon.
maybe try a post with his name for his input.

best of luck.

A regular insulated window will go miles towards good sound suppression, especially if you've already lived in the house for some time and are used to it.

White noise generators are a lot less expensive than windows.

When we lived in a New York City townhouse they used windows for Germany that were absolutely wonderful at reducing the traffic and other outdoor noise. You may be able to imagine the noise levels in New York! I don't know who the manufacturer was, but I'm sure some research may produce some sources. Good luck!