If you really want a SILENT case... than you need to look towards having no moving parts. THAT, simply, is the source of all noise.
This was my quest about a year ago. I found the radiators for the task, decided to make them my case material, but ultimately, to move water through a system... a high end system at that... I needed a pump. That meant moving parts, so my quest for NO moving parts ended pretty quickly.
Here is my post on my rig:viewtopic.php?f=14&t=62992
I recently updated (this week) with an i7-2700k running at 4.8Ghz at 1.365V, so it runs slightly cooler, but most importantly, it's completely quiet
To summarize my moving parts:
2x Noctua 140mm fans at bottom, running at less than 50% (~600rpm) rated at under 10dB
Seasonic X-850 PSU with hybrid fan that only starts turning at medium loads (silent to inaudible until system (video and CPU) are at full load
2x MCP655 pumps at level 2.5 (out of 5) - again, inaudible.
The core component of my cooling is passive - and natural convective air-flow to cool the radiators. When under load, the PSU right on top of the CPU creates additional cooling for the chipset surrounding the CPU and the memory.
But, as others have mentioned - your components will determine what steps you need to take to silence anything.
For me, I happen to have plenty of access to tools for cutting metal, rivetting and shaping the parts that I needed. So that is another factor along your path of what mods and/or creation you want to attempt.