I recently built a gaming / HTPC for ~ $1000 (see my profile). It includes a passively cooled Gigabyte Radeon HD 4850 (1 GB RAM), and uses a Noctua NH-U9B that is essentially silent.
The only thing in my rig that makes any noise is the case fans. I'm using Scythe SFF21D S-Flex fans (120 mm, 8.7 dB), but they still generate significant noise. I know it's compulsive, but I want this PC to be basically silent when it's 5 feet away, so I can read a book or whatever while it's on. Right now cooling is no issue (CPU always < 42 C, even when gaming).
I want to use 2 fans because of the design of my case. I need 1 to increase airflow behind the passive graphics card, and 1 to draw air around hard drives / CPU. Plus, if 2 identical fans are just too loud, 1 fan will still be noticeable.
connect case fans to 5V instead of 12V. That is what I have done. My PC is virtually silent except when I play graphics intensive games and 8800 GTS 512 MB fan goes up in revs as video card heats up. From lately played games it only happpens in GTAIV
What I have done I have pulled pin from fan connector from 12V position and put in 5V position. In Your 4 pin connector are 2 black wires going in(ground) in the middle, yellow wire (12V) on one side and red wire (5V) on other side. So from fan connector You pull out pin that goes to yellow wire and put it in the other side so that it connects to red wire.
Be sure to change wires on fan side on connector and not PSU. Or else You might forget about it and connect something else to that connector and blow something. If You change them on fan side of connector You are safe.
Better safe than sorry. Never know you might be connecting some old HDD/ CD at some point and if You swap wires on PSU side and plug it in something it will feed 12V where it supposed to give 5V and something might blow - connected device or PSU or both. So to keep it safe swap wire on fan connector.