Hi rupy, what is it you're trying to achieve with fanless operation?
Someone here has a signature of "fanless is pointless!" I tend to agree with this, a single good fan run very slowly, 300~500rpm, will be pretty much inaudible in all circumstances and make cooling much, much easier and more effective than fanless.
SPCR built such a thing, a Single Moving Part PC,
it's noise level is below SPCR's anechoic chamber!
This kind of PC will be large (ish), if you want a small quiet PC then look at the "heatsink cases" quite a few of which have been reviewed by SPCR. These generally only cool the CPU and you will need to use "on-board" graphics, in older systems built in to the motherboard chipset in newer ones it's in the CPU package.
If you want significant graphics power and passive operation that will be tough! For good fanless cooling size is key.
My PC is super quiet at idle, barely audible and has to be stressed hard before it increase it's noise level. Typically gaming isn't stressful enough, folding@home on CPU+GPU on the other hand gets it fairly excited, but it will pull 330w AC. Removing the CPU over-clock would help a lot.
Anyway what is it you want to achieve, if you want passive as a challenge or "for the sake of it" fair enough but if you just want a PC you can't hear you don't need a fanless one.
Hope helps, Seb