I think I might be having a similar problem. My computer boots to a certain point and then gets this screen:
It boots fine with my previous card, the Radeon 6870.
Asus P8Z68-V Pro motherboard
AMD RX 480
Corsair TX750 power supply
You said you knew your board was PCIe 3.0 compatible, so I'm assuming you have the P8Z68-v Pro/GEN3, and not the Pro?
Asus kept updating the GEN3 through 2015 (v3802), but cut the non-GEN3 version off way back in 2012 (v3603). I've seen a claim or two of the Pro variant working with an RX 480, and a few 3DMark results reporting several non-GEN P8Z68 variants (others specify Pro/GEN3). I expect it would work with an updated P8Z68-V Pro/GEN3, but I'd like see some solid answers about working/non-working BIOS revisions for the non-GEN3 variant.
I have a gigabyte ga-x79-ud3 (motherboard) and an AMD RX 480 that did not display anything upon boot.
After a bios update (from F11 in the year 2012, to F20 in the year 2014) the card worked fine.
Hopefully this will help others.
I have a similar board: gigabyte ga-x79-ud5. I was having the same problem with the RX 480. I had updated from F12 BIOS release to F13p (that some people say to be much more stable than the last version F13w that Gigabyte provides in their site). Problem solved!
Its really easy to update the BIOS on a motherboard from one of major PC building companies (ASUS, Gigabyte, ASROCK etc) My recommendation though is to do some research about updating the BIOS on a motherboard from a prebuilt PC especially if its from HP. I decided to update my motherboard several years ago and bricked my board. I called HP and they would do nothing about a BIOS update I installed FROM THEIR WEBSITE!!!
In case anyone else comes across this - I just installed an MSI Gaming X RX 470 in a P8Z68-V/GEN3 (not a pro) on the latest available BIOS (6303) and got it working. I had the same symptoms at first - boot with no video output and was worried I was screwed. The fix was to change the PCI ROM Priority setting from "EFI Compatability ROM" to "Legacy ROM" on the "Boot" tab of the BIOS.
I keep seeing several people making the claim that the problem is that the board in use is not PCI-E 3.0. This is just not the case, in fact it doesn't matter at all. PCI-E is a standard, 1.0, 2.0, 3.0. No matter what version you are using, you are still conforming to the standard. The only difference is how much data can saturate the bus at any one time. So any claims that not having PCI-E 3.0 is the problem is false. The standard is backward compatible.
Thank you, this worked for me! I created an account just to post a reply and say thank you. I searched all over the net, and was about to give up when I found your suggestion. Thank you for taking the time to post the solution, kind internet stranger!