RE: Dell XPS 8500 - Question RE: Upgrade of Graphics Card
A GTX 960 will work with the stock power supply.
Some of the 960 cards need a single 8 pin PCIe connector.
You might need a reverse Y connector that takes two six pin PCIe power and adapt to a single 8 pin PCIe. They run about $7 on eBay or newegg of amazon. Some of the cards include the power adapter.
I would replace the power supply. The one in there is probably 4+ years old and it may be time. I recently replaced/upgraded a family member's and it also cleaned up some intermittent reboot issues.
There are some solid deals right now that run cooler, quieter and are more efficient.