That's a contradiction. Quieting requires the slowing of fans which reduces cooling. The real question is quiet computing is how quiet can you get it while keeping it cool enough. For that, you need a fan controller and some temperature monitoring utility.
Find the noise and figure out how to reduce it. Likely, if you aren't running your fans at 12V, the most noise will be coming from the video card or hard drive. Get a VGA Silencer for the video card. For the hard drive, enable any acoustic options it has if it has any, or get a Samsung with NIDEC spindle motor, or enclose it in something like a Smart Drive.