ExtremeTech has an article that describes how to build a high-performance, but quiet, gaming system:

How much does this quiet gaming rig set you back? Let's take a look at pricing:

Component Product Price
- Case Antec Sonata II $100
- Power supply Seasonic S12 600W $155
- CPU Athlon 64 FX-57 $811
- Memory Corsair Twinx2048-4000PT $217
- Motherboard Abit AN8 32X $185
- CPU cooler Zalman CNPS 7700-AlCu $35
- Hard drive Western Digital WD4000YR $185
- DVD drive Pioneer DVR-108B $69
- Graphics card eVGA GeForce 7900 GTX 512MB $520
- Sound card Sound Blaster Audigy 2 ZS $72
- Sound Insulator AcoustiPack Pre-Cut Sonata Kit $69
= Total $2,418

That's not a bad price for a bleeding edge gaming system. Note that the AcoustiPack kit only costs about $69. You can find dealers of AcoustiPack kits from the Acousti website. The heat sink adds another $35, and the power supply isn't cheap, either. But you're still paying less than $2,500 for a very quiet, and very fast PC.

Of course, you can use this approach with any PC you build. All you really need is the Antec Sonata II and the AcoustiPack insulation kit. Even if you don't go with a higher-performance power supply, the Antec 450W unit supplied with the case is pretty quiet. And the AcoustiPack kit provides substantial noise reduction, even with fairly stock CPU coolers.