If both cards are used items, go for the 980. I run a gtx970 OC at 124%, so pretty close to the 980 performance, if not better in some places. There's not a game I own that gets less than 60fps at ultra settings, but most of that is because of the cpu, not the gpu.
The cpu sets fps limit, gpu sets details and resolution and tries to live upto the fps the cpu sets. CSGO I get solid 300fps. Doesn't matter if low, medium or ultra, I get 300fps. That's the limit my cpu can do. Heavily modded Skyrim, I get 60fps. Unmodded Skyrim I get over 250fps. Again, cpu limits. And my cpu is i7-3770K at 4.6GHz. All that run on Evga 550w G2.
Considering your cpu, it's lack of OC ability, you could run either 980 or 580 on a decent 550w psu. A mediocre psu, no, only the 980. And neither on a cheap built 550w psu. The Hexa+ 550w is solidly mediocre.