Computers don't need to be LOUD at all, and you don't need to spend hundreds of dollars to get them running near-silent. You also don't need to sacrific cooling to get quiet.
The problem is a simple one. Computer manufacturers use the cheapest heatsinks and fans they can buy, that will move enough air to adequately cool the system. This results in a savings of about $2 for them, and a lot of noise for you. It's easy to fix this, although it will cost a bit more than $2 to do it after the fact.
The first thing to do is to replace your heatsink. It's probably 60mm in size, with ultra-thin fins, thermal tape, and a tiny bit of space between the fins, such as this one. There are numerous alternatives. The main things you need is plenty of space between the fins, and the ability to accomodate an 80mm fan. You can find one for under $5. I would personally recomend the Thermalright ALX-800, which is easily found for $16 or less.
Install this new heatsink, but for the love of all that is holy, do not use thermal tape. It is unbelievably terrible. Using thermal paste (eg. Arctic Silver 5) will keep your processor a good 20 degrees cooler, with no other changes made.
Alternatively to a new heatsink, if your 60mm one is pretty good, you could just grab an 80mm to 60mm 'fan adapter', for about $2. I recomend buying a larger heatsink in almost all cases though.
Now, there is one fan that stands above all the rest... The Enermax 80mm. It is thermo-controlled, so it will be incredibly quite when your system is cool, and only slightly noisy when your system is very hot. Plus it's just a very quiet fan, that is cheaper than any out there. They are as cheap as $3.50 each (free s+h) if you buy 10 or more: from newegg.com. If you want less than 10 (why?), newegg's shipping prices are outrageous, so you'd be better off buying from: bestbyte.net. or you could use froogle.com, and maybe find them cheaper (not likely though).
These fans need to replace all the others in your system. Your CPU fan, your case fans (if any), and most importantly, your power supply fan. Unplug your power supply from everything before opening it up. You'll have to cut the wires going to the original fan, and leave them several inches long. Cut off the connector on the new Enermax fan, strip both pairs of wires about an inch from the end, and twist them together. Using a few rounds of electric tape will be enough to hold them together... Just make sure you route the wires so the tape won't be resting on the heatsink. It won't likely melt through the tape, but it's a chance I do not want to take.
If your soldering skills are good, I would suggest getting a 2 or 3 prong connector, and soldering it to the power supply. You can just use the connector on the molex adapter that comes with each of the enermax fans. Desolder the two leads going to the original fan, but mark which is positive/negative, and solder the 3-pin connector in there, with the correct polarity. Just cut the 3rd pin, since it's of no use.
These fans are almost the exact ones used by Enermax in it's own power supplies. Enermax has a reputation of having high quality and quiet power supplies, and now your cheap power supply will be just as quiet as an Enermax.
Connect all the fans, and turn on the system. Suddenly, the noisiest component in your system will be the hard drive. You'll still be able to hear the fans--no fan can be completely inaudible--but I can assure you they will be just as quiet as you've dream they would be.
I do not buy ANYTHING except thermally-controlled fans, for very good reason. Not because they are quiet when your system is running cool, but because it almost guarantees your system will never overheat. I live in the desert where tempuratures reach 130F degrees, and without any airconditioning at all, my systems all remain up and running, all day, every day. A half-dozen computers is a huge source of heat, but the CPUs in my systems still never get over 120F degrees (indoor tempuratures don't normally exceed 95). Not only is my solution very quiet, and very inexpensive, but it is also far more heavy-duty than the cooling system you have now. It should keep even the hottest CPUs from overheating, in abient tempuratures higher than the hottest recorded tempurature on the face of the Earth. Just as long as your system isn't in direct sunlight, it will stay cool and quiet.
This solution has more than pacified me in my search for silence. The noise from my CPU/PowerSupply/Case -fans is no longer an issue. Hard drives are getting less noisy every generation, so they won't be an issue once I upgrade my oldest (40GB) drives. The incredible noise from my DVD-ROM is another story!!!
My next journal entry will be on cool-running, yet fast CPUs and motherboards. Somewhat related to my former story on AMD power management/S2K. Expect it to appear within a week for your reading pleasure.