SFF + Very quiet = almost impossible.  
Im sorry i dont care what people have to say and what advice they give, but its JUST NOT POSSIBLE to completely silence a SFF.  My reasons are listed
1. not enough room to drop in a high CMF with low DB fan.  Only a few SFF cases even support a 120mm fan in the rear.  
2. Limited space so you can add the super quiet after market cools.  Not to mention good luck on most of the tower coolers which have proven there performance.  
Ive worked with the Silverstone, the Aspire, Aopen, and Shuttle
You want a uber quiet SFF, get a imac. It will be quieter then all the given computers in SFF.  
You dont like macs?  then get a Shuttle.  Hands down shuttle will pwn all the other SFF's in quietness.  But dont expect high overclocks
Still insistant on getting a uber quiet SFF.   :\       Here is my latest creation of my friend who wanted a super cute little SFF thats uber quiet.  
The Build:
Aspire Q-Pac.  Case
Fan: 120mm Yate Loom + 2 x 80mm Coolermaster  inside front helping air across the power regulators and ram. 
Mobo: your choise make sure its MATX
CPU : again your choice make sure u match the CPU with the board   
 
  
Hd: Seagates 7200.10 series.   <--- best quiet / performance ratio hands down
PSU: Seasonic M12 or Antec Neo 500W  <Modular PSU's arent as great, and theres plenty of studies on this fact. 
But in a SFF, without modular, your gonna have one hell of a mess inside>  
Video: Almost anything except a 8800GTX or 7900GTX series.  Your gonna hvae card length issues with the Qpak since it uses a Full PSU
Ram: Doesnt matter, just match your board
Cooling:  Corsair Nautilius 500 External Watercooling with DD Maze4 GPU block + DD Maze NB Block.   
<This kit only costs 149.99 with and the 2 extra blocks are 49.99 + 29.99 respectively>  its a 200 dollar cooling option but the box is small enough to fit on top of a shuttle even and with quick release, transportation isnt so much of a hassle. 
You could try to custom a setup inside your qpack. And i recomended Qpack because IVE tried and failed HARD and putting H2o in a silverstone.  The Qoack also has enough room inside to put a Swiftech Premium Kit, if your okey with 1 optical. But ur gonna need some experience in h2o if you want to put put the entire kit inside and not have it external. 
<The external is almost noobie proof cuz corsiar did a good job in this kit>
<your not going to get quieter then this unless you decide to watercool a shuttle instead> 
Cuz its on water, you should get decient overclocks. And it will perform better then a tuniq tower if your cooling the CPU only. Replace the CPU block with the new Dtek fusion or Apogee GT  <--- i promise u'll cool better then ANY AIR COOLERS PERIOD. THERE IS NO CONTEST PERIOD.   
My friends Comp booted at 2.4ghz on a E6300 Othros Stable with coretemp's never breaking 50C on cpu, and her 7900GT never broke 40C oc'd to 550. I did however replace the CPU block the kit came with to an Apogee so your performance may be slightly lower.
She only plays WOW at 1280x1024 so she didnt need anything more powerful then a 7900GT.  Also 7900GT's still has one of the best power/heat/performance raitio out of all the cards out there. 
Ummmmm all in all, get a P180 and go air for the same price as going SFF on water.  It will cost the same if not a little less, produce the same noise, but u'll have more room in the case do whatever u want.     
 
  
My .01 cent