I'd go with the idea of replacing the fans, I have used Quiet PC in the past, they are quite expensive but carry good stock and always quote the noise levels.

I use alot of Zalman stuff, in fact it's 2 of their 120mm fans that keep my server room cool and quiet. They still shift a good amount of air but are almost silent. I have found the same thing with most of their products, none of them I have had failed to keep the stuff cool while keeping the volume to a minimum.

Cheers

Cris.