I am a regular member at SPCR (
http://www.silentpcreview.com)
If your computer is not too intense, it will run fine with two 120mm fans contrary to belief, with NO INTAKE OR EXHAUST.
Four things you need to do :
1.) Swap your power supply for a Seasonic S12-430 (a given!)
2.) Swap the stock P4 heatsink for a Thermalright XP-120 (if it can fit), paired with a Nexus 120mm fan (the quietest fan on the market)
3.) Swap the stock heatsink + fan on your video card for a Zalman ZM-80D or a Zalman ZM-80C (depending on your video card; 9800+ and equivalent will require a D), paired with a Zalman ZM123 fan bracket also paired to another Nexus 120mm fan.
The two Nexus 120mm fans would be undervolted to 7v using an inline resistor or a rheobus, your choice. These are cheap, doesn't really matter.
Remove all your intake/exhaust fans. Unnecessary; I have a Lian Li PC75 (your case's bigger brother), no intake/outake, P4 3.2C @ 4.0, X800 Pro at 520/500, same setup as above and no overheating issues (3.2C is at 54C during gaming).
4.) After doing all this, your computer will be dead silent; the last thing is the hard drive. If you don't want to hear seeks or idle noise, you should invest in either a SmartDrive 2002 or a SilenX Luxurae (I have a Luxurae).
Good luck with your silencing endevaours, and if you have any more questions feel free to PM me or reply to this thread.
- Adrian Kwok.
EDIT : I read some of the posts above. You can undervolt a fan as much as you want, but there -will- be motor noise, I guarantee it. The Nexuses have the least out of all fans I have tested (I have over 10 120mm fans of different makes, Panaflo, SilenX, Coolermaster, etc). Panaflo is -ok-, nothing great.
I would choose the new Coolermaster 120mm LED'd Sleeve Bearing fans over it in a flash, and over the Nexuses too if I were strapped for cash.
Note : if the 120mm Nexuses are too expensive for you, I suggest the 120mm Coolermaster SLEEVE BEARING fans (TLF-S12-EB), as they are very inexpensive and at 7v are acoustically equivalent to the Nexus, with no bearing noise. They are available in only LED versions, unfortunately.
EDIT AGAIN :
Stay away from Vantec Stealths, SilenX fans, or anything NOT Nexus/Panaflo. Make sure if you go for the $8 Coolermasters that you go for the SLEEVE bearing, S12 model!