I'm building a system with an i5 3570 processor, SSD + 5400RPM HDD, and a Radeon 6950.

I can crudely estimate that under load, my system will probably draw less than 400W of power, but I rarely play games so it will probably be idling most of the time drawing closer to 200W.

I am wondering if there is any benefit from an acoustic/silencing perspective for me to get a PSU that is more than I really need.

I'm torn between a Seasonic X560 and Seasonic X760. There is about a $55 price difference between the two, which seems like a steep premium, but for my system would it actually be more quiet to get the higher-rated power supply? I'm guessing that with an X760 the fan would spin slower when I'm operating at higher loads, right?

I'm eager to hear the feedback from those who may have tried a variety of PSUs under different circumstances. Thanks!

At less than 20% load, the Seasonic fan for X-Series 560/660/760/850 does not spin at all, and then starts spinning at a constant rate until 50% load. Above 50% load, then it starts ramping up geometrically:
http://www.seasonicusa.com/NEW_X-series ... 60-850.htm

I think you are over-estimating your load. I draw 66 watts at idle for the following:

Asus Sabertooth P67 motherboard
Intel 2500K CPU
8 GB RAM - Corsair
MSI R6850 Cyclone PE/OC
Intel SSD - 120 GB
Western Digital Green Drive - 1.5 TB
LG DVD/CD Burner
Asus Xonar DG Sound Card
Seasonic X-660

For your computer you are looking at max 100W idle power usage, more likely sub-90W.

Where did you get those numbers? Your idle power consumption will be less than 100W. And your load consumption might be less than 300W. (GPU uses about 160W and the rest maybe 130W).

I'd get a PSU that is passive at low loads. The best PSU seems to be Kingwin/SuperFlower 550W model.

http://www.silentpcreview.com/Kingwin_L ... um_LZP-550

The Seasonic X-Series PSU's he is considering are passive (no fan spin) at low loads (below 20% of rated output). For the X-560, that would be no fan spin below 112 watts, which is not quite double the idle load.

But the 550W Kingwin won't spin below 400W at all. The fan is just a fail safe device.

I'll pile on: <90W idle and <300W load, where 20-25W of the power at idle is due to the gpu.

+1 for the Kingwin modular LZP or the non-modular AP-550. Similar (and sometimes lower than) pricing to Seasonic gold for platinum efficiency, silent operation at idle and many load conditions.

There's sort of a Goldilocks zone for PSU specs:
- You don't want to spec the power too low: get into fan ramp up / noise
- you don't want to spec it to high: crappy efficiency at idle and low loads.

1. I was responding to your post about fan spinning at low loads. I correctly pointed out that the Seasonic does not spin at low loads either.

2. The Seasonic does spin in "silent mode" at about 16 db (according to Seasonic) between 20-50% load. Obviously, 16 db is not completely silent, but unless once is operating a recording studio, I don't think one would hear it at normal distances. Above 50% load, it starts to ramp up (in what you might call fall-safe mode). These numbers will be adjusted automatically by the PSU if ambient temperature exceed 25C.

3. Seasonic does make two very simular X-Series PSU's with no fan at all (X-400/460) if that is what is needed, but very few people would hear the X-560/660/760 fan over the noise of GPU fan, music, or whatever, if their PSU was running above 50% load.

I was reading somewhere that the x6950 drew nearly 250 Watts at load! I'm hoping that this is an overestimation... Or perhaps the figure I was seeing was for the entire PC, which makes more sense.

At any rate, the impression that I'm getting here is that I can save some money by buying an X560.

Ah yes, that was power at wall socket. So way more than I need to budget for PSU capacity.
http://www.bit-tech.net/hardware/2010/1 ... -review/10

At idle even on an X560, the PSU should run in silent mode or fanless.

Those numbers are still way too high. According to the link you provided, my system with 6850 GPU would run 131 watts at idle, when I only use 66 (measured). The discrepancy is the CPU. Their test setup used an Intel Core i7-965 Extreme processor with TDP of 130 watts, whereas my 2500K has TDP of 95 watts. But your 3570 CPU has TDP of only 77 watts, and idles much lower than the Intel Core i7-965 Extreme (which was not built with any power saving features in mind).