Depends on the gaming PC. Anywhere from ~200W to 1500W. The most common is probably 350-450W.
You have to choose everything carefully, and it does add a fairly big chunk of change, $100-$200 depending.
Case
You want to choose something that isn't covered in mesh and preferably has sound deadening material. Fractal Define R4 is the case of choice for this, but the Bitfenix Shinobi is a good budget option.
Video Card
This is generally the loudest part on a gaming machine. The most quiet stock coolers are (in order of quietest->loudest) the Gigabyte Windforce, MSI Twin Frozr, and ASUS DCII. There's some that come in and out of these rankings from PowerColor, Gainward, and Zotac. They are usually inconsistent, changing, and region specific though.
Case Fans
Another important factor as this is what is *always* running. Some suggest a fan controller, but fan controllers have become all but obsolete with new 120mm fan technology. You want low spinning, low dB fans. The best of the bunch right now are Corsair AF Quiet Edition, Bitfenix Spectre Pros, Noctuas, and NoiseBlockers. Low speed Scythe and Yate Loon fans can also work just fine, and are a bit less expensive.
PSU
PSU choice is also important, as you'll hopefully get something that is a hybrid and has enough overhead to never really get hot at full load. If you want to totally maximize, you would want a PSU that can deliver double what your total system draws. For a modern gaming PC, you'd want a Corsair AX750, Seasonic X750 (Corsair AX is a rebranded seasonic), or Seasonic Platinum series. Depending on the hardware, you could go up or down in watts.
CPU Cooler
Also an important choice, but, contrary to the way reviews are run and how most people recommend such parts, it's the fan included that should really be under scrutiny. A Coolermaster Hyper 212 with a pair of really good fans like the Corsair SP Quiet Edition, Bitfenix Spectre Pros, or Noctua NF-F12s will be nearly inaudible compared to a Noctua NH-D14 with crappy CoolerMaster fans. So, just get a decent heatsink tower and pair some good fans with it. Or, grab a Noctua heatsink that is compatible with your platform.
Watercooling
This adds hundreds of dollars to the expense, but on my Shinobi XL, the loudest part is a Spectre Pro which runs at 18.9 dB(A). It's inaudible even in my super quiet rural house. The only moving parts are the fans that are attached to the radiators and the pump. The pump itself is extremely quiet. There is no difference in sound between idle and load, it's always whisper quiet. There's no CPU/GPU fans spinning up and down.
http://www.neogaf.com/forum/newreply.php?do=newreply&p=40857027
It leaves you no room for further upgrades down the line, but if you're not trying to build a 'gaming' PC, then that's pretty much what I'd use to build something like an HTPC.
Swap the case with a Bitfenix Merc or Outlaw. Grab a Corsair Builders Series 430W, just to make sure you have a nice new PSU. If you don't need a ton of storage, you could even swap the HDD for an inexpensive 60GB SSD.