Adding a video card
What video cards can I use to upgrade my system
The OptiPlex 755 offers energy-efficient performance, stability and choice in a mainstream business desktop.
Dell has annoying proprietary motherboards, but this one still allows small form factor PCI-e GPU’s.
If you can explain what you want to use the GPU for, we can select a fitting one for you.
For HD video playback, an AMD HD4550 should be fine. For light gaming a Nvidia 750 is about the max I would trust the crappy power supply in those things with.