I want a quiet but powerful PC

You can google "The Silent PC" to find out how. This is a well worn area and even your noisy machine could be quieter to silent.
Bob

You can probably go with a passively cooled video card. That's what I did. The heat sink is rather large, however. It's a midrange based on the ATI 4850 chipset but nvidia makes equivalent. I went with 120 mm fans of better quality and with speed control. I've an i5 quad core using stock Intel fan and heat sink. Nothing is special about the case but you can go with all kinds of acoustic dampening if you wish. A big consideration will be where you want the case to sit. I prefer below my desk as it's quieter there but it will gather more dust. Good luck .

Here are some of the rules I have followed to build quiet desktop machines over the past 15 years.

Larger fans can move the same amount of air at lower speeds than smaller ones. Look for a case that supports 120mm or larger fans. I use brushless fans that have a speed control switch built in. Setting the fans on low should move enough air to keep the computer happy and lower the noise.

If you get a noisy fan take it back - try a different brand or type! Fans can resonate in one case and be quiet in another.

I have fans in the front of the case that push air into the case and fans at the back and top of the case to pull air out of the machine. This pushes the noise out the back and top of the case.

Look for rubber gasket material to place between the fan and the case mounts to minimize fan vibration noise.
I cut my gaskets from an old bike innertube and used rubber cement to stick them to the fans. (ok I am cheap)

Next look at the powersupply and the fan inside. Again a larger fan can move air at lower speeds helping lower the noise again. Again you can insulate any vibration between the powersupply and case with gasket material.

Look at the space where you will place the box. Avoid placing the case in a space where the back is enclosed - I used to keep my computer under my desk and it was loud because the noise was amplified by the location - almost like the desk was resonating the sound. I suspect placing egg crates, foam or even adding carpet might have absorbed the sound.

Your video card might add noise. If you can live without serious PC games try to find a fanless card or use the built in graphics.

Organize and route internal cables to keep them away from fans and improve airflow.

Keep the machine clean. Dust will build up on fans, in ports and vents. The dust will eventually cause the fans to go out of balance and add some noise. Be very careful when cleaning your computer case. Check the dust buildup on the CPU cooler too.

#1: Find an efficient power supply. Find a power supply that is highly efficient (+75% as rated on the box. Look for 80+) which runs cooler, and has a large fan that spins slower, thus making less noise. Not coincidentally, more powerful power supplies have bigger fans and are often more efficient. If you don't play games you don't NEED more than 350 watts, but a 450-600 watt PSU will be quieter because it isn't straining to get the power to your components.

#2: The CPU fan that came with your Intel or AMD chip sucks. Why? because the fans are tiny and have to run faster to cool the processor. Get an aftermarket heat sink with a big (80mm or larger) sleeveless fan. Once again, the larger the fan, the slower it can spin to do the same amount of cooling.

#3: Big case fans. Small case fans are louder because, once again, they must spin faster to move the minimum amount of air to keep the inside of your PC cool. Unfortunately, case designs and fan sizes go together...you can't replace small fans with larger ones because the fans must match the holes in the case. Thus, you want a case with large fan openings and large slow fans in the brackets. Many manufacturers have standardized on 120mm fans for quiet and efficiency; some fan mounts are made for 140mm fans, that's almost 6" across! Conversely, while you want maximum cooling, a case that's all perforations, mesh and vents is just going to let noise out. Get a case that has a solid front (except for maybe one fan opening) and as few side and top vents as possible, to keep noise inside where it belongs.

#4: Your video card. This is the noise source that you have the least control over since most people generally don't replace the fans on a graphics card (though I did, with a noisy ATI 9800 Pro, see below). Each manufacturer is different and you just have to read about the different brands once you have an idea of which card to buy. The more advanced the card is, the hotter it'll run, and the fans will be correspondingly louder. But even between models (let's take an ATI 4850, a midrange gaming card, for example). A Diamond Multimedia model may be quieter than a Sapphire model solely because of the maker's choice of fans. In general, the brands that are less well known spend less thought on noise, but they often cost less as well. Fact is, when you're not a gamer, video cards are much less noisy, so all the better for you. Get an inexpensive card and it may be quiet enough.

If you follow all these steps, you can still probably build a midrange PC for around $600-700, especially if you steal the drives and such from your old PC. But what if you DON'T want to build your own PC?

If you really intended for your question to be, "well I don't want to build a PC by myself; whose brand is exceptionally quiet?" then I have a weird suggestion for you. Wait for it...

Buy a Mac.

Yep, a Mac. Apple computers, for all their faults and priciness, are among the quietest computers you can buy. Each Macintosh model is designed from the ground up to be noiseless, and unless you get a loud aftermarket video card (the Apple-branded ATI 9800 Pros were really well-known for this), are virtually silent in day-to-day operation, gaming included.

If you don't want to be a switcher, the "new PC" rule to follow is, the cheaper the box, the lousier the noise abatement. Look at HPs, Dells, whichever, but if it's a low-end PC, it'll have low-end parts in it and less time spent on engineering. There's a reason people who buy better PCs are happier with them: it's because the designers went a little further to do good industrial design. And that benefits you, the user.

Good luck!

Noise, generally generated by fans in your PC's case (although sometimes, as a harbinger of bad news, it can come from your hard disk), is a function largely of the design of the external case.

I would recommend one of the high-end Antec cases (model 900 and up). These are a bit pricey when compared to other cases (although still only a minor expense in the total cost of a PC), but they are very nearly completely silent and offer superior cooling with multiple fans, which gives you not only a quieter but a more reliable PC.

Hi Royeo,

In my experience (28 years) fan noise usually comes from one of the following:

1. Cheap fans. You would think that a fan is a fan, but there are huge differences in the quality of fans out there. As stated in previous posts, the bigger the fan, the better, but don't go cheap. If a fan is going to be noisy, you'll know as soon as you install it. If it's too loud, take it back and find a different one. You can compare the noise level ratings on the packaging, and that will help, but the best noise measuring tool you have is your own ears. The same fan may run quiet in one computer and sound much louder in another.

2. Dirty or Worn Fans. I've worked on computers that had so much dust, dirt, grease, and gunk built up I was surprised the fans turned at all. So it's a good idea once a month to open up your case and blast it with a can of compressed air. If your computer is in a particularly dirty/dusty environment, you may want to perform this maintenance more frequently. Grease particles, second hand smoke, and particularly dusty air environments are highly detrimental to all of your computer components and should be avoided if possible. When a fan that was once quiet begins making noise, you should replace it immediately. Before you do, make sure the screws holding your fan in place are secure and dampened with rubber washers.

3. Faulty Fan Speed Setup. Many motherboard setup programs have specially designed controls that adjust the CPU fan speed automatically. You'll want to be sure that the CPU fan you've selected for your setup is compatible with receiving those instructions. If not, the fan will run at full speed all of the time. You should also take care when building your computer to properly mount the Heatsink to the CPU with quality thermal grease like Arctic Silver.

4. Overheating CPU. Obviously, one of the most frequent reasons for high levels of fan noise is high fan speed. When your CPU gets hotter, the Fan needs to run faster to help the heatsink dissipate the heat. There are some new and clever products on the market now that appear to be making great strides in low noise heat reduction without the need for complicated Water Cooling Systems. One I tried out recently is from Ultra. This system incorporates a device called a Thermo-electric Cooler (or TEC) together with a Heatsink/Fan assembly. The TEC isn't new technology, but they always had a problem when the TEC lowered the temperature to lower than room temperature, causing condensation (of water) around the CPU. This new technology only runs the TEC when CPU load exceeds a predetermined load level, and turns it off (using only the standard heatsink/fan apparatus) when the CPU load is below the predetermined level. I think it's really cool technology and has been proven effective in keeping the CPU temp down as well as the noise level.

The model I was reviewing was the Ultra ChillTEC 939/AM2/AM3/775/1366 Thermal Electric CPU Cooler.

Hope this helps. Good Luck.

If you like the way your present computer is performing then you might consider replacing the case itself; here is the link for a Cooler Master Silent Cosmos, it's heavy, but built for silence.
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119214