If you want the quietest solution, Acoustifan 92mm will cost you like 20$ from a few select places. I have heard that panaflo distributors are giving all the online resellers a hard time. I have gotten this by calling SVC, Sidewindercomputers.com, and one other place. Frozencpu.com and Directron.com still sell the Panaflo 92mm Mediums. However, at a 12$ pricetag. Frozencpu guarantees an RPM sensing fan.
If you want a bit more airflow and bearable noise, get the 92mm Panaflo BX from frozencpu. You can turn down the voltage on this if 30db is too noisy.
After 12 various fans from thermaltake, all of which i fell for their db rating, I cannot recommend them.
If you are replacing your 80mm case fans as well, get rid of the ones that either, sound horrible, or are not very effective with moving air.