Kasper Jorgensen said:
Your PSU is just fine. No need to upgrade.
The R9 390 PSU requirements are 600 to 650watt PSU.
I wouldn't say you would have to upgrade as your unit is pretty good quality, But I am afraid you may be on the PSU's limits even at stock.
You can still however overclock, The PSU will end up powering down randomly if it exceeds its limits, It won't damage anything if it does, But I do not know how much headroom you would have before it will do that, Or if you overclock and get bad performance drops, then that is a sign of lack of power.
Now AMD knows their cards and knows what they will draw, Like I said you have a good unit, But its hard to tell how far your unit will go before it trips and shutsdown, I'd say you would be better off upgrading to at least a 650 watt if you plan to get the highest overclock that you can. It's a power hungry card, unfortunately.
That's just my opinion.