Its NOT just the card, I would have to buy a card AND a monitor because NVidia refuses to support anything jut their Gsync module. This module adds hundreds to a price of the monitor for what AMD does for nearly free.
I would suggest getting a free sync monitor. A good friend of mine just got his a week ago and he LOVES it. He went from a standard 1080p60Hz to an 1080p144Hz with freesync and he doesn’t stop telling me how great it is xD Also his aim in overwatch drastically improved too.
So I’d say get a freesync monitor and depending on the resolution/refreshrate of the monitor keep your 390 or get an vega 56 card as soon as the partnermodels are available.