RE: Dell XPS 8500 - Question RE: Upgrade of Graphics Card
My opinion ( just one ). I would not upgrade the graphics card or PSU
If you are not gaming, then a GT 640 is fine for day-to-day workloads.
If the system is stable, then I would not upgrade the PSU either.
If you are looking for an upgrade, put in a SSD.. a 500GB Samsung Evo 850 or even a 1TB. Clone it from your current system and enjoy a much snappier system.
as far as the PSU, the 550W EVGA G3 is plenty for your system plus a GTX 1060, even a GTX 1070, but again for the usage you have a graphics card upgrade is not really needed.