Your gonna have one of these 3 power supplies and Dell did a real disservice to move that computer from anything but an office computer.
Not impossible to overcome with after market pigtail wires to convert regular power supply to Dells configuration so you can add standard power supply and correct power you need + 6 and 8 PCI-E power plugs
I would replace the power supply before installing a dedicated video card. The factory power supplies in those are not very powerful, probably less than 300W, which typically isn't enough to drive a dedicated video card.