if im use 6 x R9 390x how much PSU i need ?
your custom PSU is 1700watt ? 2500watt ?
With most of these things, you should profile a single card first, and then extrapolate from there. From what I've seen with the 390's (not X), they're running about 275w per card, and my CPU overhead is about 60w and cooling is more on top of that. You should refer to whatever PSU you're using, but most of the server ones are more efficient as they approach 100% load (as opposed to consumer ones which peak at about 80%). So a DPS2000BB would more than work (at least in my situation). The biggest issue I've run into, and it's definitely solveable but you should be aware - is that you'll need to provide at least 3a of 5v per card (on the PCI side), as the PCI side provides both 12v and 5v. Most of the breakouts/server PSU's don't provide 5v, so you'll need to get a step-down converter to handle it, but it's not that big a deal.
I've just been going through this whole exercise as I've been working on my own GPU mining case using the same PSU's I use on the bitcoin side.
