You should be using DP to DP, assuming your monitor has a Display Port connection. In addition, a high quality 4K DP cable (at the 1.4 rating, not the previous 1.2 rating which is only good for 4K at 60 Hz) is required. Do not convert DP to HDMI for a gaming monitor. It goes without saying that you need all power ports connected to your PSU, using separate PCIe cables.
As Albert Einstein said, "I could have done so much more with a Big Al's Computer!".