I'll trade performance for silence. I'm thinking of implementing a passively cooled PSU and VGA - and a passively cooled procs.

The Mod I have in mind is anchored on using a towering heat pipe cpu cooler (like the Asus Silent Square Pro)....minus the fan.

My case is Antec Lanboy (the rear 120mm I'll use as intake to blow on the cpu heatsink; while the front 120mmm fan will serve as the exhaust fan. Those are just about the only 2 fan that will run.

So my question is, will underclocking an e6400 for example, will make it cool enough so as not to require its heatpipe cooler not running a fan?

there are osme specially designed fanless heatsinks for example (presumably for sale in america) http://www.dabs.com/productview.aspx?Quicklinx=3QZM&SearchType=1&CategorySelectedId=11023&SearchTerms=fanless&PageMode=3&SearchKey=All&SearchMode=All&NavigationKey=11023
A cpu like that gives out quite alot of heat, so if this is for a new build an amd dual core would be easier to cool without a fan.

The scythe ninja and infinity are MADE for fanless operation.

Maybe you'll need to undervolt it too.

If you are going to be using the case fans they would be noisier then Third party fan sytem (not stock). I have one of these on one of my builds, it makes next to no noise.
http://www.newegg.com/Product/Product.asp?Item=N82E16835186134

Same as for your graphic card, since you didn't list your graphic card here's a list of silent coolers
http://www.newegg.com/Product/ProductList.asp?Submit=ENE&N=2010110576+50002107&Subcategory=576&description=&Ntk=&srchInDesc=