Fractal Design Define R4 ATX Mid Tower Window Computer Case Black $59.99 plus shipping

Not sure how the side window affects noise dampening (vs non-windowed case) but this is a really nice case. I had the R3

Does the window add that much more noise to the case? How effective is their noise dampening material?

If you mean the difference between the windowed and non-windowed, there certainly is a difference, but whether it is noticeable or not is going to vary person by person. Acoustic padding and thick side panels do a really good job blocking high-pitched noise, so if you have anything that higher pitch than say the air movement caused by a fan, I would definitely recommend using the non-windowed panel if you are going for an ultra quiet computer. For some that pitch of noise is not noticeable, but for others is very annoying. Personally, I really like getting my personal computers as quiet as possible so I tend to use non-windowed side panels whenever I can.

Sorry I can't give you any actual numbers here. The difference between windowed and non-windowed side panels is on my list of things to investigate, but unfortunately we have not had the time to get hard data yet.