My rig is pretty much built for silence, and I'm a lurker on the
SPCR forums. Computer noise is for me one of the worst sounds in the world.
case: P180
mobo: Asus P5W-Deluxe (the downside with my board is that it FUCKS UP wi-fi. I get lag stutters all the time despite 100mbit fiber connection).
cpu: C2D E6600 with a Scythe Ninja heatsink
ram: 4gb of something I forget the name of.
sfx: Audigy 2
gpu: Geforce 7900 GT with a Zalman VF700 CU
two 21" samsung monitors @ 1600x1200 each.
and a hacked version of windows xp which has removed almost eeeeeeevverything (the cd is 130 mb) except the very bare essentials. runs much smoother. Next best thing to linux I guess (which I wish they would make games for).
In the case I've got 4 nexus real silent fans and wind tunnel setup that is nearly identical to this
http://www.silentpcreview.com/article672-page1.html
I can only hear my computer if everything is DEAD silent, and I mean dead silent. If even my window is open and I can hear leaves rustling in the trees outside I have to put my ear to the case to even notice that it's on.
My next rig I will probably be able to be put in a sort of closet though, so I can have louder components and overclock it without hearing it anyway.