Before spending any more money, do a few simple tests on your computing environment. Very possibly you are getting harmonics from the computer desk/stand/table and not realizing it. Your furniture can actually amplify the sound with sympathetic vibration. You can either isolate the furniture or the computer. It's most simple to use something like a rubber mat or small piece of carpet to absorb any vibration that the cases are passing into the furniture.
If that doesn't work, and you have the freedom to do so, try moving the case onto the floor (and a hardwood floor is equally capable of "some" harmonic vibration, although it's unlikely, unless the house is quite old, or the floor installation was a poor quality job (in other words, some of the boards in the floor are LOOSE.) You can also obtain a wide variety of fan speed controlling devices, some expensive, some not. With those, any individual cooling fan creating sounds you personally find objectionable can be adjusted to sound differently.
The two noisiest items, potentially, on a modern PC are the CPU's hsf unit, and the video card. Few of the cooling fans on motherboards make much noise, and the average power supply fans aren't usually among the noisier items. The hsf can usually be quieted by the inexpensive route of getting an adapter and a larger fan; the small 60 mm fans on a lot of hsf's spin very fast, and make a lot of very high pitched whining noise.
Video card hsf's are the (noise) offenders so often that various aftermarket replacement cooling options are popular; you have already been reminded to look into the Arctic Cooler brand. They are supposed to be quite good.
:beer: