I purchased one of the server PSU's from minersource.net (was very professional and fast - would highly recommend them). I think for long term, that is by far the best solution, assuming you can stomach the noise - which is the only drawback I see. Highly reliable and high efficiency, and an excellent value - only around $80 to power two PSUs is great.
I'm running into a problem though, in that I might be in a situation where I need to run 2 S1's short-term and need another PSU. Its a long story involving an eBay buyer who has to wait to pay me etc..
I have a Corsair AX750, which is a gold PSU that I use to run my desktop - that I was considering trying to use to power the two S1's for a week or two - depending on how long this eBayer takes. It has 62A on the 12V single rail, and enough PCI-X connectors. It has also been supply two 7950's running overclocked, along with an overclocked i7 920 - faithfully and reliably. Link to the stats:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139016If I was going to be running these two until they aren't profitable to run anymore, I would defientely buy another server PSU. I'm not really excited though about purchasing such a dedicated PSU - I suspect it might be difficult to resell on eBay after everyone is using the new 1Th units that come with a power supply.
What is everyone's thoughts on trying to use that unit for a few weeks? It still is covered with several years of warranty, but don't wanna blow up a unit because of something silly.
Thoughts?