Power from your machine:  the original power supply from Dell OptiPlex 390 mini tower is 265w.  It provides up to 75w to power graphics card on the PCIe x16 slot. Power requirements for AMD Radeon R7 250:  it's a 65w dual slot card and doesn't need external power connector.  Suggested power supply of 250w. I'd say your machine should be able to run the R7 250 GPU.
so i just want to know if my optiplex can run R7 250 2gb GDDR5 gpu my optiplex is mini tower and has core i3 2120 [NAME] 3.3  it has 6 gb ram and 1 Tb hdd idk how much watts that my PSU can give to the pc the main question is is it able to run the gpu?
is this gpu compatible with optiplex 390