My goal was to build a solid modern PC, fast but not ultra-fast or exotic, with a video card that wouldn't choke if I threw a game at it. With Silent PC Review as my guide, how silent could I get?
With newer components delivering more PC power for less energy, wattage could be minimized. Fewer watts in the box meant I could get by with less air flow than before.
Start with the Antec Solo II case and the Seasonic SS-400FL. The Solo II, aside from being generally quiet, is optimized for a fanless power supply, since the power supply mounts at the top with a vent above. Aside from being silent, this means any heat from the power supply doesn't end up inside the case. So we're well on our way to silence and a box that doesn't need much cooling. The 400 watt potential will be far more than enough.
I got a mid-level Z77 motherboard and a 65-watt-max Ivy Bridge quad-core CPU. Add an SSD. There's the heart of a system, and it will idle at very few watts. (Feeling extreme? Get a 45-watt max CPU. I didn't go quite that far, and I didn't see the 45-watt models available as I went to market.)
For silent mid-range graphics, add a fanless video card, the Sapphire Ultimate Radeon 7750. (I see PowerColor just came out with a similar fanless "Go! Green AX7750".) These will draw something like 40 watts max, and idle at a few watts.
With so few watts in the box, I dare to mount the slowest, quietest fans. My choice: Noctua NF-S12B ULN, using the ULN resistor. It doesn't move much air at its lowest setting, but then there's not much heat to eject. Can barely tell if these are on. I mount two of them on the front intake, then I remove the very good exhaust fan that came with the Solo II and replace it with a NF-S12B ULN. With two fans blowing in and one blowing out, I should be getting positive pressure in the box; that's good in that the components will be cleaner. The entering air will pass through the intake filter in the case front, and will push out through other openings (better than sucking in air and dust through those openings).
It's complete overkill, but I stick a Noctua NH-D14 heatsink on the CPU. This is a fan favored by overclockers. Attached to a low-power CPU, a huge heatsink like that could probably do the job passively based upon air moving through the case. But, setting aside the included fans, I stick another NF-S12B ULN in the center. This fan doesn't push much air and it's not intended for this purpose, but I'll use it anyway. (Seems that the Noctua fans aren't the favorite of Silent PC Reviews. Could be you can do better. I've been okay with Noctua in the past, and they give their fans a 6-year warranty and claim a big MTBF number. I doubt I'd ever try to collect a warranty on a fan, but that's a statement of manufacturer confidence.) By the way, I found the NH-D14 easy and relatively fool-proof to install, and I'm a bit thumb-fingered.
For bulk storage, I added a Western Digital Green 2TB disk. The WD Green drives are cool and quiet; not the fastest, but fast enough for me.
So, bottom line is my system idles in the 30-40 watt range (about 70 watts lower than my last PC) and has four NF-S12B ULN fans. I've pretty much achieved my goal; can't tell whether it's even on unless the room is silent and I get close to the PC. I guess the only thing keeping this PC from being perfect is I haven't found a pair of silent speakers.
When I put the PC to work, the power draw rises. May have seen it touch 100 watts. Still should be okay. I've been watching the temperature monitors; they idle in the 30's or 40's and I haven't seen anything rise above the 60's. (Useful program: HWMonitor, from http://www.cpuid.com/softwares/hwmonitor.html
; it knows about Ivy Bridge.) Haven't yet tried a torture test, but I don't really torture the machine much in real use; I just want it snappy.
So I spent extra money replacing good fans with quieter fans and buying an expensive heatsink, and I didn't reach for the fastest CPU or GPU, and I'm not overclocking. But if this works out okay -- so far, so good! -- then I've got my money's worth.