A old motherboard like P5QPro with a P45 Chipset can suport 2x RX580 in crossfire?
In theory yes, but non UEFI motherboards will often have trouble with UEFI only cards like the RX 580.
you should first consider buy a decent motherboard with ram, cpu and a 1000w psu, to do the rx580 crossfire and btw if you already running a rx580 on that motherboard, put another graphic card is not going to make any diference, you cant even handle the full power of one graphic card.
I just bought one.I was just curious. And with this two cards mining should be fine?
for mining you dont use crossfire, but you need that psu anyway.
I bought a ax860i more than 5 years ago.
5 years ago is old now PSUs these days dont even last 5 years. I replace on a cycle of 2-3 years as quality to quantity is getting worse I HAd 2 decent PSus die in 18 months. So I go with a 2 year cycle as the load they will be under U end up causing a fire or cooking your card & void all warranties. An upgrade on Motherboard is needed for mining to btw, you need a motherboard with usb3 ports minimum at least for a basic mining build, going off a few good mates who have built a few now anything below usb2 support is a waste of time. IF you wanna build a mining rig need a good quality PSU gold rated minimum of good brand & PCIE 3 supported board with usb3 support to.
Not entirely true what you say about PSU age, i own a Corsair TX850 PSU, with almost 10 years old and its behaving like a champion, well i do maintenance, once a year i tear it down to clean and look at soldering, the psu is handling a AMD HD 6990 OC with a AMD Phenom II 965 x4 at 4.2Ghz OC on a Thermaltake 850i water cooler, 4x2GB Corsair Dominator and a Asus Crosshair III Formula... i use it 24/7 in server with Windows 10 Server and Linux Debian.
Now about the Topic, For what the guy whants he didnt understand what i wrote about new System and the PSU, but who am i to confront and confirm anything, let him buy the new GPU, when the system dont work he ask again.
Basic answer is no you motherboard wont support with the PSU you got without running into many issues that arnt worth considering. More problems than shelling out some decent $ to build a basic mining rig. I wouldn't bother build a mining rig as it is as they are a gimic that's going to crash & burn very soon the outlay for the gain isnt worth it unless you got a decent budget to start with but if your going that far may as well build an allround PC for your self dont dedicate it to mining isnt justifyable.
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