Ok , Well i didnt read through ALL the idea's , one thing I didnt see mentioned was to grease/oil the fans you allready have ( if possible ). If they have a sticker on the center, peel it back and if underneath the sticker you see an open hole with the fan blade pole in there, put in a drop or two of sewing machine ( 5 in 1 ) oil . This way if its the bearings that are causing the noise, it'll stop.

I agree with the other posts but the quietest fans with the most airflow I have found are the Noctua 120 mms if you can afford them. I spent a good chunk of money on a few of the 80mm for my sff job and they were worth every penny. They are practically silent but blow a good bit of air.

Undervolting! A few of my fans are running undervolted, and are SILENT. They still push enough air for my liking, so it's win-win.

If this has been suggested before, sorry for repeating :)

If your fan guards are the kind with tiny holes or even the kind with small slots, you can make the fans quieter by cutting them out and replacing them with the kind that look like barbecue grills.
http://www.svc.com/finguargril.html
Front fans don't need any guards.
This will do 2 things increase airflow, and slow the fan down 1 or 2 hundred RPMs.
I know this sounds counter intuitive, but although not true DC fans, the speed is still effected by load, the more airflow the more load. hence slower speed and quieter.

Edit: the little holes are a source of noise also