If the R9 380X turns up at the end of October as rumored, then that would be my choice.
I'm expecting it to give close to GTX 970 performance at a much better price. If you have a 1080P screen then you won't need more power than that.
Otherwise if you want max power for 1440P and up then the R9 390 has a lot of raw power, but it might stress out the 600w power supply. I usually recommend good quality 650w power supplies for the R9 390.