Unlike standard ATX motherboards, the uses:
: Includes one PCI Express x16 slot for discrete graphics and typically two legacy PCI slots (depending on the specific chassis/form factor).
While it has a PCIe x16 slot, its compatibility with modern GPUs is limited by the system's proprietary power supply and physical chassis size (Mini-Tower vs. Small Form Factor).
This motherboard likely supports specific generations of Intel or AMD processors, depending on the chipset. For accurate information, consulting Dell's documentation or product support pages is recommended.
If you are looking to purchase a replacement or upgrade components, consider retailers like Tech Atlantix for the motherboard itself or Newtown Spares for specific OptiPlex 780 parts. Dell OptiPlex 780 Service Manual--Mini Tower Computer