First things first

[maalitehdas]

To make a silent computer from a scratch, it's always a compromised solution between

1. dB level
2. performance
3. heat (long lasting)
4. power consumption
5. price

Before you go on reading, you should put these in a right order for your personal needs, since you just can't have all of them.

1st: I would start with the required performance. I would think about 2-3 years time, and build a system with minimum requirement in that time for my personal needs. Lot's of DIY builders also leave some options for upgrades later, expecially when choosing a motherboard, but I wouldn't think about that since all the parts are getting cheaper or better in that time anyway.

2nd: I would choose the main components (MB, CPU, GPU, MEM, HDD, optD) which minimally meet my requirements, thinking about power consumption and price (in that order). No need to think about the sound lvl yet exept for the HDD section.

3rd: When the main components are found, I would choose the power supply just enough for those (+20%). You can use this calculator to help you choose the needed watts: http://extreme.outervision.com/psucalculatorlite.jsp and SPCR's recommended PSU list for minimum dB's here.

4th: Then I would go for choosing the cooler section. Heatsinks for CPU and GPU are a must, optionally for NB, with help of SPRC's heatsink recommendations. Water cooling I would leave out from the cart, since with the price of it you can change all the other parts with lowest possible dB's.

5th: Just enough fans (0-4), zero fans for only very low performance system that are possible to build with passive cooling. SPRC's recommendationswill help you to choose your fans with lowest dB's. Aim for just enough airflow with few undervoltaged fans. Also choose better fans for replacements of the always noisy stock fans.

5th: I would choose a silent case that have room for all my parts. Antec have been popular for long with it's possibilities for different builds and room for the biggest heatsinks and ducting. Nexus, Silverstone, Zalman and Arctic cooling are also worth visiting. Remember to get rid of the lousy stock fans delivered with the cases!

6th: Putting everything up is up to you. Airflow can pe pushed from down / front or sucked from top / behind, or use both (i wouldn't). Air can be tunneled inside the case with foam / pipes / ducts / plates. Most important is that there is moving air, expecially close the heatsinks.

7th: Choosing the rpm's of the fans is very important. Fans can be undervoltaged in automatic, manual or solid way. You need to considerate your list again that you (hopefully) putted in an order in the beginning. Slower rpm's will mean less dB's and more heat. Do you want it more silent or to last longer? Give it a thought again.

I hope this thread will help you in your plan.