All the fans come with noise range by dB. The marketing tactics the companies give spot the lowest noise where the fan runs at idle. You should make sure the upper noise level because some of mid-range CPU fans have bad reputation. $40 to $50 fans create more noise than $4-$5 or stock one.
What I did, I upgraded a CPU and added 2 GiB memory. Previous CPU was AMD Athlon 64 LE 2.8GHz, single core. I just found a Dual Core 4.8GHz on a internet shopping mall where I live. It cost me about $40 for the CPU, $24 for two GiB memories. The system is 64bit Dual core and 3 GiB memory.
After I plugged in the dual core, and two GiB memories, the average CPU temperature sits at 28C to 29C which has been at 44C flat most of time. So, I basically reduced 14C which is very impressive number. Whenever previous system was overloaded, the CPU temperature went up 59C to 60C. I think It was running the CPU fan full-speed.
I probably lower the voltage of CPU fan and the power supply fan, and add extra fan in the case later on.
Dual core CPU reminded me http://en.wikipedia.org/wiki/ABIT_BP6 which offered two Intel Celeron slots.
I had fun with it.
PS: Do not disconnect the power supply fan. The unit will blows out within few minutes. I lost four-year old one while I was playing around. A good learning experience.
