There is almost no chance the power supply in that Dell is going to be good enough for a GTX 960. Dell puts power supplies in these workstations just powerful enough for the hardware they include to save money. The power supply they use in their workstations is not meant to support a GPU.
There is a good chance that the power supply included in your Dell doesn't even have a PCIe power cable that is required to plug into the 960.
The best GPU that is going to have a shot of running on the stock power supply is a GTX 750Ti since it can draw it's power from the motherboard instead of needing a dedicated cable from the power supply. Even then you may, or may not have enough power with the stock power supply.
You really need to get a...