OP, sometimes you can receive a dud. It happens. But it is not wise to discard a whole gpu line just because one faulty card.
Although just introduced cards can have design flaws because of rushed production.
We have seen this with various AMD and Nvidia based designs. Just read through the forum.
GPU cards are manufactured by the hundred thousands and sometimes you receive a defect one. It happens.
A quality 500W PSU is more than enough for any rx480 card.
If an 850W psu cannot handle a humble RX480, that power supply is defective or just a bad design.
I for example have a season 520W supply and have played for hours without any issue.
I hope that the card you initially got was a manufacturing dud and that you can have fun gaming times with your new card.
As a side note because you are a new pc builder:
Some equipment gets damaged because of unproper ESD handling.
Any pc component is an esd sensitive device.
It is good practice to own and use an esd bracelet.
This esd bracelet will make sure you and the largest metal surface of the pc case(which is Ground) have the same voltage potential.
Also, do never touch the pci-e connector of gpu card with your bare hands.
Always touch the metal mounting bracket first and then the other side of the gpu card.
This way , the ground surface of the gpu will have the same potential as you.
Through the bracelet you have the same potential as the pc case, minimizing any possible electrostatic sparks.
With ESD it is basically that every conductive surface holds an electric charge(skin is also conductive) . And when two different electrically conducting surfaces meet while having a different voltage potential, this difference will equalize, usually through a spark which holds enough energy to damage for example the gpu chip or the pci-e bridge chip on the motherboard.
But ram, the cpu and any component might get damaged.
The lower the humidity is of the air in your environment, the worse esd gets.
https://en.wikipedia.org/wiki/Electrostatic_discharge