The CPU with best ratio: power/power consumption/price/noise

For good info on quiet computing, go to www.silentpcreview,com.

A CPU makes no noise, it is the fans that make noise.
If you want SILENT computing, you can have no fans, and your capabilities will be limited.
If QUIET will do, then there are lots of options. By quiet, I mean that the PC is almost inaudible from a few feet away.

Start with a quiet case.
The Antec Solo(or designer500/P150/sonata-plus550) is about the best around.
http://www.newegg.com/Product/Prod [...] 6811129018
It has sound deadening panels, baffled intakes, suspension hard drive mounts...
With the stock fan on low, it is very quiet, but you can always replace the fan.
If a 120mm fan turns at less than 900rpm or so, you can hardly hear it.

A good oem tower cooler will cool any cpu out there decently. Passive cooling will not work well as a rule, but if you put a slow turning fan on the cooler, you won't be able to hear it because it is buried inside of the case. The Xigmatek s1283 will do the job well.
http://www.newegg.com/Product/Prod [...] 6835233003

Look for a 45nm cpu. They are much easier to cool.

Look for a quiet PSU, and get one a bit larger than you need. Corsair is very good. A PSU that operates at about 1/3 of it's capacity will not need to ramp up it's cooling fan.

If you are not a gamer, there are several fanless vga cards around. Any will do. If you want to play games, you need a vga card that keeps fans low when it is not being used much. Look at the SPCR link for some suggestions.

Most hard drives are relatively quiet. Interestingly, the velociraptor can be one of the quietest, but it probably does not have the capacity that you need.

---good luck----

The E5200 is probably the coolest running desktop CPU currently available, not counting Nano or Atom here, just 'regular' desktop chips: http://www.xbitlabs.com/articles/c [...] html#sect0

The X2 4850e is not a bad choice either, but being 65nm will consume a bit more power. Its a bit cheaper though. I don't think you can passively cool it (or the E5200 for that matter), but a decent HSF with the fan at a minimal setting should do the trick, and would be just as good as 'silent'. Many mobos have dynamic fan settings that adjusts the fan speed according to the CPU load/temps.

You can choose a silent heatsink from here:

Top 5 heatsinks:

http://www.frostytech.com/top5heatsinks.cfm

http://www.tomshardware.com/review [...] 67-12.html

Top 5 Low Profile Heatsinks on Frostytech

http://www.frostytech.com/top5_low [...] tsinks.cfm

AMD has some 45W CPUs too like the 4450e.

AMD Athlon X2 4850e & Gigabyte 780g - from Tom's ...

42w @ idle
73w during HD playback

http://www.tomshardware.com/review [...] 85-14.html

Hmm, was just browsing THG's articles and this is an odd one: E7200 + G31?!

http://www.tomshardware.com/review [...] 39-13.html

30W idle, 60W full load... damn. Granted the G31 is a weak IGP but in terms of system power draw thats impressive, especially with a relatively powerful CPU in the E7200.