As said above, you really can't put any high end cards into your system without hitting a major bottleneck. If you don't want to change your motherboard, you could look for a Lynnfield quad core eg. Core i5 7xx or Core i7 8xx, though I wouldn't pay too much for one, they're getting long in the tooth and will bottleneck the really high end cards currently on the market unless you put a pretty heavy overclock on the CPU.
R9 390 is a pretty power hungry card, and I would recommend more than 500 Watts for the power supply. Even a really good 500 Watt PSU would be pushed close to its limits. The GTX 980 isn't as power hungry and would run on a 500 Watt PSU if it was a good quality unit.