dell optiplex 390 pci e max wattage

It's not a matter of wattage as this forum post explains. Notice the comment regarding R series AMD cards. Keep in mind that is an old system approaching 9 years old and technology has changed greatly in that time. It may be time to consider a replacement.

Stock power supply in 390 790 990 is LESS THAN 300W and therefore 75W cards are not supported.  390 790 990 is a bios system that requires MSDOS VESA video mode 103 for the F2 CMOS Setup.  RX Series cards do not support legacy MSDOS or VESA or XP and require UEFI to initialize.   Non Dell OEM cards require Secure boot OFF and CSM ON. 

When installing an after-market graphics card into a Windows PC with UEFI secure boot enabled, the system may not boot.  This is true for ANY PC not just Dell.

http://nvidia.custhelp.com/app/answers/detail/a_id/3156

  There is no UEFI or CSM in a 390 so from both hardware and software its not ever working.

Recommended card for Older Dells up to current is GT1030.

https://www.youtube.com/watch?v=2y32mBdZHJY

Works in all models EXCEPT USFF and Micro because they do not have slots or 75W PCI-E slot power.

The Dell GT1030 for Tower models is 8CCF1.  The Retail parts are also available from Dell.

So the 1030 will work just fine with a 250w psu