I'm a very big fan of quiet PC's. Back in the day when money was limited and I tried to build for the cheapest price possible, my machines were quite noisy. Then, I purchased a Dell for home which was very quiet. After getting used to having a powerful machine that was quiet...I was unwilling to accept a noisy PC.
For me, quiet PC's are the result of
#1). Quality cases and materials. Many of us here are fortunate to have nice cases, but they make a huge difference. My quietest cases are my Antec Sonata II's...but my Antec P182 is not much noiser. Also, the rubber grommets and such within the case dampening the hard drives and other case vibrations makes a significant difference.
#2). Quality power supplies. Again, many of us here are fortunate and have nice power supplies...but the fans in the cheapo PSU's are often the cause of much of the machine noise.
Coolers and the like can make an impact. But these are often necessary..especially when overclocking is being performed. For me, I like to stick with the stock cooling on my intel CPU's as it's very quiet...but it obviously does limit how far I can push the box.