The Dell 520's manual can be downloaded here
. The PC's motherboard optionally has a VGA Video Connector (integrated)
as shown on Page 1 of the manual. If yours is equipped with this option, there is no graphics card installed - it is built into the motherboard itself. It can not be removed or separated from the PC.
Your PC may have a a separate video card installed into one of the expansion slots on the motherboard. Page 1 also shows this installed below the USB 2.0 connectors as the PCI Express graphics card
Here's an internal view:
The video card in the picture is driving two standard analog VGA monitors (blue cable connectors) through a DVI -> VGA adapter (white and clear devices) this is connected to the video output connectors. If you look closely at the edge of the PC in the picture (directly above the USB & network cables) you should be able to see an unused VGA Video Connector (integrated).
Most computer BIOS systems will automatically disable any built in video capabilities of the motherboard when a separate video card is detected. Check the manual for additional info about these BIOS settings.
I hope this helps & good luck!