Yes, you can run PCI-E v3 cards on a PCI-E v1.1 and up board as long as you use a card that is recogized by the Bios in the primary PCI-E slot. I was running a HD 7850 PCI-E v2.0 card and two RX 480's with powered USB risers on a Dell XPS 700 motherboard that had PCI-E v 1.1 slots. The speed of the slot makes no difference for mining. The newer PCI-E specifications are able to deliver more power through the slot, but as long as you use powered risers the card can get enough power without overloading the motherboard. The problem I had on the XPS 700 was the BIOS would not recognize the PCI-E v3 cards on Post. I had to use the HD 7850 PCI-E v2 card in the primary PCI-E slot to be able to access the Bios. Once in the OS, all the cards were recognized and mined.
Thanks! So the main thing is I leave the RX580 in the primary pci-e x16 slot, and put the 5600XT in a riser. I hope it will work.