Look read again I did wrote down clear the GTX1070 is to much for his PSU, why here coming back to this type of video card. This on youtube is an perfect sample on how to use that adapter, no rocket science nor advice for the use of the GTX1070.
if it can run the GTX660 that was in the Dell option list, it can run an GTX1660ti easy. For checking this I use an PSU calculator. https://outervision.com/power-supply-calculator
PSU Power needed,
GTX1660ti 274watt on 12v, PSU advice total 416watt so this is below the 460watt, from your Dell PSU.
The biggest Vcard this XPS 8700 could have in the Dell option list was the GTX660 here it is 297watt on 12v, PSU advice total 433Watt
So the GTX 1660ti even with the needed 2*6->8 adapter, would be a safe choice, the same on the GTX1060 btw. The 6 copper wire from the PSU can handle the power thats going true it.
For mr Sopwith as I wrote down: go to the Dell forum, do a search like “XPS 8700 videocard” or “XPS 8700 graphic card” and you will tons on info about that upgrade. Be sure it will fit in your small PC case.