Found this article about the RX 480: http://www.digitaltrends.com/computing/rx-480-power-issues/
From what I've gathered from the post if you OC it you run the risk of drawing to much power from your motherboard. Which after awhile could fry your PCI slot if you are lucky. I'd watch out for this and at the very least not OC the card at the moment.
edit 2: Found a bad PDF...
edit 3: Found a reddit post with the guy claiming the 480 is causing issues with certain motherboards:
https://www.reddit.com/r/Amd/comments/4qfwd4/rx480_fails_pcie_specification/
I also saw some posts on the AMD forum as well: https://community.amd.com/thread/202410
*Disclaimer:I'm not sure how reliable the first post is. Only thing I can do is assume there is some truth. The second(AMD) one seems alot more reliable.
edit 4: Some links provided by @SoulFallen that migh be useful for you guys to form an opinion
edit 4: AMD released a statement to anadtech:
link: http://www.anandtech.com/show/10465/amd-releases-statement-on-radeon-rx-480-power-consumption
edit 5: Seems like AMD has posted a driver update that supposed to shift some of the power load off of the PCIe slot in order to bring it within specification, however, this does put your 6 pin out spec... So people with old/crappy PSU probably should avoid OCing the GPU, but AMD has setting for you in the Radeo settings menu in order to fix this. Anandtech also ran some performance checks on it so I'd definitely check out the link.
Source: http://www.anandtech.com/show/10477/amd-posts-radeon-1671-drivers-fixes-power-consumption-issues