I've got the Gigabyte Z87X-OC mobo (it rocks.) with ASUS R9 280X cards (4 of them) I'll share my experience -
Currently I'm having issues connecting the 3rd GPU. When running with 2, everything is fine and stable. when connecting a 3rd one, system becomes unstable.
I'm on Xubuntu, and I think it is related to two things: 1 - the PCI slots that i'm using and 2 - the drivers.
When I used the first 3 PCI slots (PCI 16x, PCI 8x and PCI1_4x) the bios kept rebooting the system. Then I changed the slots to PCI 16x, PCI 8x and PCI2_4x and the system booted correctly.
Then I tried AMD Catalyst drivers 13.11beta (which worked fine for 2 cards) but when the 3rd card was connected, the system became unstable. (GUI freezes, cgminer doesn't start, some more weird issues that sometimes solved after reboot and sometimes not).
So I tried using Xubuntu repository fglrx drivers, and it recognized my cards as Radeon HD 7900. With this I could run cgminer for few minutes before one of the cards just stop mining, but the system is very unstable. for example, if i'm closing cgminer, I would need to reboot the machine before being able to re-run it again.
I tried uninstalling these drivers and wanted to give a shot to AMD Catalyst 13.12 (not beta!) but my OS crashed after I reinstalled the fglrx drivers.
I'm using Xubuntu 13.10 btw.
Any recommendations on which PCI slots to use for it to work properly? Does anyone else had these issues? Any advice is welcome, as I'm out of ideas.
Also, is there someone here that run 3+ R9 280X cards successfuly on Linux? Which gpu drivers are you using?
Once I figure it out, i'll post my solution. thanks to whoever gives a hand to the subject