I would consider myself knowledgeable after spending a a good deal of money recently for a silent upgrade for my PC (new case, cpu fan, gpu fan, case fans and motherboard)
Your first priority should be the Noctua NF-S12B. It's more expensive than Scythe in general, but it's the cutting edge in terms of silence and performance. However it launched little less than a month ago so it may be hard to find still.
If you cannot get hold of this, second priority is the Scythe Slipstream 1200 RPM with id code SY1225SL12M. If you get this, 'underclock' it to 800 RPM or remember to set BIOS settings to auto adjust fan speeds in silent mode if you can.
Finally just slightly behind the Slipstream on third priority is the Scythe S-flex series. Again choose the medium model and 'underclock' it for maximum efficiency and flexibility.
All of these fans are extremely silent yet very effective.