Due to the OCZ Powerstream 520's 88% full-load efficiency, it generates little heat and gets by with very little fan speed. While the fans that come in the supply aren't any great shakes, they spin slowly enough to indeed allow it to be one of the quieter supplies of true substance. If you play with the fan fitment you can sure make it very quiet. Crotale's sustained a 616W output in testing a DI rig, so it's probably worth the effort.
I don't think the Silverstone is as quiet, but is certainly a powerful supply. But it makes more heat than does the OCZ because of efficiency, and makes due with the same 80mm exhaust fan size. Since it must spin its like-sized fan faster, the Silverstone is louder.
The Seasonic is a great supply, nearly as efficient as the OCZ and has a great fan choice and fitment as well as an effective fan speed controller. The only knock on it is the dual rail topology, and while it is clearly powerful it has yet to be proved that a P-D 9xx won't be limited by its output given enough clock and voltage. The OCZ 520 is a safer bet from an ouptut perspective, although neither is exactly in the cheap seats.
I agree though, you have to have a really low noise floor before this is going to make or break the results. Consider the lengths I have gone to quiet my sig machine:
- 1.5mm thick aluminum case with acoustic foam dampening material
- Yate Loon 120mm case exhaust running at 12V, 1250rpm/47cfm
- Scythe Ninja heatsink w/another 120mm Yate Loon at 12V
- Aerocool VM-101 passive video card heatsink, Coolermaster bolt-on aluminum memory heatsink, custom machine TR NB1 w/Everflow 40mm fan at 3700rpm in place of VM-101 heatpipe clamp plate
- Northbridge fan replaced with second 40mm Everflow at 3700rpm
- dual Samsung SATA2 drives for storage (quietest made)
- single 74GB Raptor for boot (quiet FDB spin noise, moderate seek noise)
- all hard drives suspended in 1/2" wide Wallyworld elastic
- fan grills cut, front case inlet drastically enlarged and fitted with home-AC filter material
And once you get all those ducks in a row, the power supply becomes a big issue. I use an Antec TPII-480 fitted with a .35A 5-blade high-pressure Yate Loon 120mm fan from a FSP350-60PN. The fan is sealed via a Vantec gasket and the grill moved to outside the case lid. The "fan only" output of the Antec TP supplies is an output of the same temperature-dependant controller circuit that governs the internal fan's speed. I use this to drive a Top Motor 120mm fan that is fitted to the case's inlet, so that as the power suppy gets hot the increase in air demanded by its increased internal fan speed is met by the comensurate increase in case inlet fan speed.
If you are prepared to split every hair, power supply heat production, fan size, and fan choice are crucial factors that must be leveraged to produce first-rate overall results. But your description of your machine does tend to indicate a noise floor as high or higher than a power supply of average noise level generates. My machine is very quiet from a few feet away at idle, and only increases the power supply and inlet fan speeds to audible levels under sustained 100% CPU or GPU load. And even then, it's still the quietest computer in the room (and decisively the most powerful at the same time).
The lesson to be learned here is that it takes a total-system approach to get anywhere near silence, and the better supplies like the OCZ and even moreso the Seasonic S12s are already essentially there.
