My pc is literally inaudiable - the only thing making noise is my graphics card (6600GT - with a stock cooler).
I have very good temperatures as well :D
The way i've done this is not necersarily an expensive case.. but the fans and heatsink that you use to cool the components..
For example: i have a Cooler Master Centurion 2 - it cost £30 including vat - without a PSU, then i have a 480W ThermalTake Silent PurePower (with adjustable PSU fan speed) which cost me £65 at the time, but u can pick one up alot cheaper than that now :)
If you're getting or have got a AMD64 system i recommend:
HSF: Artic Freezer 64 Heatsink & Fan
Case Fans: Vantec Stealth Fans (available in 80mm/92mm/120mm)
Graphics card cooler: Artic Cooler Range
I have an Artic Freezer 64 for my heatsink & fan, using Artic Coolers thermal paste that comes with it (Very good stuff), i use 2 x 80mm Vantec Stealth Fans (80mm), and once it is released i will get a Artic Cooler VGA Silencer for my Nvidia 6600GT - as the retail stock cooler is v.loud.
The Vantec Stealth Fans run at a constant 21Decibels - which is very low for a case fan - but it pushes huge amounts of air through the case (which is great - low sound - high cooling capabilities).
The Artic Freezer 64, is quite big, but is compatible with all 939 / 754Pin AMD64 motherboards, and is a doddle to fit, just clips straight on..
The fan is built in - and on the side of the fan (not on top)..
The only noise you'll receive thats down to your case is vibration or rattling components, so just buy a case from a well known manufacturer such as Antec / Cooler Master e.t.c
Get a quiet Powersupply, quiet case fans and a quiet but good heatsink and fan :)
I took a Decibel Reader from my local Audio shop where a friend works, and i took 15 measurements around the computer (in and out side) and the average noise its making is 24Decibels (a high of 26 inside the case), readings from outside the case (near the fans) were max of 23-24DB again, but thats merely because of my graphics card, everything else is 22DB or lower..
Also, fans create vibrations.. whether you have a good case or not, so get some Acoustic matting, this is sound proof matting that u can surround the edge of your fans with, so it doesnt have immediate contact with the metal of your case - cancelling the vibration, also when you get a case try and get a case with 120mm Case Fans, these make the least noise whilst giving Maximum airflow.
Any metal grills or grills of any form that are surrounding the fan cut them out, as the metal grills create turbulance from the airflow - as they are in the way.
Use Rounded IDE Cables for your Optical drives (CD/DVD/DVD-RW e.t.c) or any IDE / UIDE hard drives you have - and make sure your 4 & 20 Pin Powersupply cables do not obstruct your heatsink and fan.
Hope this helps :good: