This forum page has quite a bit of information on the PCIe slot.
Hi there, I'm getting a 1650 super for my dell OptiPlex 990MT its either going to be the RoG STRIX version,  ASUS TUF version, or a msi but they require a PCIe 3.0 x16 for port connection does the 990MT have this also i have already upgraded to a 600w psu and have a six pin ready
1650 super