This link has some good info and recommendations.

Here is a link that might be useful: Sound-Abatement Windows

This is quite after-the-fact, but I can tell you that STC varies enormously by product type. If Pella is saying 27-35, then 35 would probably be for something heavy like an outswing single door with midrail and laminated glass, and 27 would be for something relatively leaky like a horizontal sliding window.

Construction quality makes a huge difference as well. A luxury manufactured window like a Loewen casement starts an STC 33 with standard IG, and can get up to STC 40 with Loewen's acoustic glazing offerings. Of course, Loewen is easily double the price of Pella.

Still, truly "soundproof" starts around STC 50, and only products built specifically for acoustic attenuation can meet that criteria. Also, your typical 2x4 wall only has an STC of about 33, so once your windows match the STC of your walls/roof, windows cease to be the primary source of sound infiltration.

Stanislaus Ting, why even bring this up after almost 10 years?

I was looking for information on Pella STC ratings for comparison, and this thread was actually much more informative than Pella's website. The data doesn't appear to have changed over the past decade, so it's still current.