ASRock Z87 Pro4
will I be able to put two same cards in this one? (it says it has "1 x PCIe 3.0 x16, 1 x PCIe 2.0 x16, 2 x PCIe 2.0 x1" I'm not sure)
As long as they are single or double-slot wide, 2 can fit.
Ok. I was unsure because it says "1 x PCIe 3.0 x16, 1 x PCIe 2.0 x16" not "2 x PCIe 3.0 x16", but if it doesn't make a difference (for hashcat at least, not gaming) that's good.
Keep in mind you may need to leave a space as well between them for cooling depending on your cards and chassis.
Thanks for noticing, I may let the case be open. Cards will be 280X (I know its a rebranded Tahiti HD 7970, but what matters is it has 2048 Unified Shaders, Hawaii has more but not much and is too expensive).

Cards didn't improve much in the last years, only the number of Unified Shaders and so did go higher. Imagine it would be not in 28nm but in 1nm, then it would be a nice speed improvement Big Grin (improving the architecture is still important for speed increase). Time to switch to at least 14nm anyway but it's easy to say.

Although Nvidia has better Linux driver I go with AMD because they have a more open OpenCL approach instead of the more kind of proprietary (and slower) CUDA.

Found out that cards don't even fully use PCIe 2.0 x16 and so PCIe 3.0 x16 is just kind of a marketing thing.

Interesting: "Double precision performance of Hawaii is 1/8 of single precision performance,[21] Tahiti is 1/4 of single precision performance, others 28 nm chip is 1/16 of single precision performance."
So Hawaii got worse on some important points (but it's not half as slow).