Well that's what I get for ordering open box........ Finally all the components arrived a few days ago including the replacement case, got it all put together and oh shit!! I need a power cable for the graphics card........ No PCI-E power cable........ FUCK!!!!
Contacted Rosewill requesting a PCI-E power cable be sent, hope it doesn't take them for freaking ever........
You are not properly informed. If you had read a review of the RX570, you would know the 75 watts of the PCI-E slot would not be sufficient for that card. Also, if your PSU doesn´t have a cable, it is possibly very old and not suitable for such a graphics card.
This RX570 has a 8 pol power connection:
Yours maybe not, there is a variant with two 6 pol connections as far as I know.
As for the PSU, be sure you have at least this values for the 12V line, the other stuff is not very important, CPU and GPU get their energy from the 12V line. You see 12v x 37,5a makes 450w.
See, the RX570 consumes up to 200 watts alone:
AMD Radeon RX 570 Power Consumption - Tom's Hardware
I have another advice for you while gaming (or not). The AMD software contains "Radeon Overlay" and you can activate with CTRL, Shift and o. It shows you details about your GPU and CPU like temperature, fan speed, ect and fps. You can also customize it.