In the past few months I have been on a quest to silence my case. It gets a little annoying trying to sleep with it on. Here's what I had:
antec sx1030B, with 2 case fans and an antec 300 watt P/S
Mini super orb (2 fans, approx. 5000 rpm)
ibm 75 gp
ibm 34 gp
What I found was that the bottom fan on the super orb was creating 30% of my noise. I disconnected it, undervolted and underclocked the CPU (recently I brought it back up to 800, it's running fine), ripped off the chipset fan, aimed my gamma 28 at the chipset fan, and disconnected the two case fans.
After all that, the machine is still louder than I'd like, but a large part of that is the hard drives, and they're not so easy to replace on a $20 budget.
If you want quiet, look into the gamma series of blowers. They move a lot of air, and are wisper silent. Also, there was a 5.25" fan system called the twin turbo or something like that that moved tremendous amounts of air at only 26 dBa (very quiet). It's over at 2cooltek for $18 (justcooler) here: http://2cooltek.safeshopper.com/45/34.htm?851