Greetings!
Most GTX 650s require a 6 pin connection from the PSU. Most OEM systems do not have an extra 6 pin extending from their PSUs for they are unnecessary for the original system design. Your PSU is probably a 240 - 300 Watt variant for these are common in Dells. My guess is that you will likely need to upgrade your PSU in order to add a GTX 650 to the rig. The big concern with this however is that Dell has a tendency to use proprietary PSU connections which may make it impossible to add a different PSU for connecting the new PSU to the actual motherboard would not work due to the proprietary nature of their OEM wiring design.
Here is a form chat from Dell's community forum on the topic...