Generally AMD cards age fairly well, given the newer features of the 480, i would be tempted to go for one rather than the 390X, although i would definitely wait for the aftermarket cooled and clocked cards with hopefully something to address the power worries... It does already seem the 480 performs exceptionally well in DX12, i can imagine a better clocked and cooled one will be quite the card.
The other option if your are not desperate for a new card (like myself) is wait to see the RX 490, its listed on AMD's website, so it should make an appearance at some point, this will probably be the card aimed at replacing the 390X. We have no other details other than its called an RX 490 though.