I've gone for a totally quiet setup undervolting all of my vans to 5v-7v.
I opted for the
Scythe Ninja so I could go for either passive cooling (100% silent) or 5v undervolted 12cm fan (virtually silent from 8" away) or full speed 12cm for great cooling during overclocking (still relatively silent).
If you're a real silent PC enthusiast, opt for either
Nexus 12cm fans (one of the best rated silent fans for undervolting and performance, but pretty expensive). Or you can do what I did and opt for the
Yate Loon 12cm Fans (they are essentially the same fan as the Nexus, just generic and not undervolted like the Nexus). These two fans are virtually dead silent when undervolted. I opted for the Yate Loons because they're about 1/4 the cost of the Nexus.
To undervolt your fans, you can either hardwire them, but thats a pain in the butt, especially if you're not really good with soldering. I dropped $10 for this undervolting module
Cooler Master Aerogate III which can control up to 4 fans. It feeds you temperature in 4 different locations in your case such as CPU, VGA, Hard DRive, as well as RPM. You can range your undervolt from 5v to 12v reducing your fan speed (and consequently, fan noise). Pretty much makes every fan in your case silent.