dell optiplex 390 DT supported graphics

Hardware Corner here suggests a GTX 1060 or GTX 1650 Super but you would need a SATA to PCie power adapter. Alternatively you could go with a GT 1030 or RX 6400, which do not need the adapter, but in any case it must be a low profile GPU.

@JOcean​ thanks mate

in addition to JO comments, the DT has a 250w oem psu so it is good for low profile GTX 1050, 1050ti, or 1650 that do not need a 6 pin PCIe connector (note some maker add OC feature to the card and add 6 pin socket to the card, which is above/beyond Nvidia reference).

if the gpu is not for gaming, Dell oem RX550 and RX640 LP are also options which are less expensive than RX6400.

390 DT board supports dual slot width card

https://i.dell.com/sites/csdocuments/Shared-Content_data-Sheets_Documents/en/Optiplex-390-Technical-Guidebook-101311.pdf

@amor100​ Happy we could help out!