I connected the Dell P2720D monitor to the HDMI port of the iGPU of the XPS 8300 as you suggested.  The monitor works great!  I increased the resolution to 2048 x 1080.  You are brilliant!  Thanks again!
I have an XPS 8300 i7-2600 with 8 GB RAM. I purchased it in 2011 with an AMD Radeon HD 6770 graphics card. A few years ago I connected the graphics card to a Dell S2718NX monitor via HDMI. A year or two ago the screen started turning blue or white or striped after some hours of use. I mistakenly thought the problem was in the monitor, and I purchased a Dell P2720D monitor, which supports display port and HDMI, to replace the Dell S2718NX monitor. The video problem persisted with the new monitor, and I realized that the problem was not in the monitor but in the AMD Radeon HD 6770 graphics card. (If I had read the Dell Community bulletin board, I would have learned that the problem was in the graphics card and not in the monitor!) I removed the AMD Radeon HD 6770 graphics card, and I connected the iGPU on the XPS 8300 motherboard to the older Dell S2718NX monitor via VGA. The newer Dell P2720D monitor does not support VGA (from the iGPU), only display port and HDMI. The video problem vanished, confirming that it was in the AMD Radeon HD 6770 graphics card. I am satisfied with the video from the iGPU. It works fine for Skype and Zoom and for photo editing. I do not play any video games. However, I would like to purchase a graphics card that supports HDMI and/or display port so that I can use the Dell P2720D monitor and potentially sell the older Dell S2718NX monitor. One graphics card I found is the Dell NJ0D3 AMD Radeon HD 7570, which supports display port: Will this graphics card work on my XPS 8300? One user reported a problem trying to use an HD 7570 in his XPS 8300 in March 2016: I also found a couple of ATI Radeon HD 5450 graphics cards. These do not support display port, but they do support HDMI: Which graphics card would you recommend to me to connect my XPS 8300 to my Dell P2720D monitor?
XPS 8300, graphics card for Dell P2720D monitor