en.community.dell.com/.../19611037 has many examples.
Sasonic X-650 is my go-to standard, and is more than enough for any single graphics card.
Do you already have the GTX 780? If not you might consider a GTX 1060 6GB gaming edition. The newer cards are more power efficient and I think a GTX 1060 is roughly equal to a GTX 780.
A GTX 1060 will work with the OEM power supply. Some GTX 1060 cards need a single 8 pin and if the card does not come with a dual 6 pin to single 8 pin reverse Y adapter, they can be found on eBay for about $7.00 USD shipped.
The other downside of a GTX 780 is the heat from the card. I ran an overclocked GTX 770 in an xps 8700 and it got hot. I cut the front of the case and added an intake fan and it helped, however after upgraded to a GTX 970 it ran much cooler.
The GTX 1060 is slightly faster/more powerful than the 970 and it produces even less heat than the 970 so it should do fine with the stock 8700 case.
good luck. post pics on the thread above when you do the upgrade.