DonkeyOatie :
You are using a less than top grade power supply and you are pushing it to and beyond it's maximum at 500W. Where did you get the 500W from? The 260X has a 115W TDP why do you think that you need 500W? An NVidia GTX750ti is an alternative, better is some places, worse in others, and has a 60W TDP.
First about the 12v-rail, don't worry about that. It has a 6-pin PCI-E power connector that will be used which always is set up to give 75w of power.
@DonkeyOatie: Dude you need to do some more reading. These PSUs are not top dollar units, but they are excellent units for budget systems first and are highly recommended. I can't even take a guess at how many computers I have seen get recommended to use them on this website in budget builds, over 40 times at least and I am not here a lot.
Second, the PSU's max power rating is 430w, not 500w.
Third, nothing about the system he has indicates that it will ever be pushed up to or past 400w, let alone 500w.
Fourth, The Nvidia GTX 750Ti beats it in all places:
http://www.guru3d.com/articles_pages/nvidia_geforce_gtx_750_and_750_ti_review,11.html
http://www.anandtech.com/show/7764/the-nvidia-geforce-gtx-750-ti-and-gtx-750-review-maxwell/13
Finally, the TDP for AMDs cards is never given. It never is for any GPU really, so its not good to assume 115w TDP or 60w TDP for those cards. A major reason for this is because it can change from card to card, for example some GTX 750 TI cards have been ran with a 6-pin power connector to allow heavy overclocking. While power consumption and TDP are not the same measurement they are related so TDP will be much higher on these cards than those without the extra power connector.