The motherboard of a Optiplex 390 is only a x16 1.1 slot if I am not mistaken, so it really doesn't have enough bus to use the card efficiently. Also, what is your OS and Operating System? And what is telling you XFX Support is not secure?
i also bought rx 460 for my now old upgradesed 600 euro oem pc i hope only ebay digital to vga vill work i vwill make video on youtobe how it runs on old athlon cpu vs low graphics shity Playstation 4
Beter not use oem old del computers they use lots of power becouse slow dualcore intel cpu and one of these died for my brother
Or beter use some used laptop new ones are expensive and also low qualty whit taxes