I'm using a R9 280X from toxic (OC) and I run everything in high and I have some lag.
To be more specific, during the beta 3 I had no lag whatsoever, the game was very smooth and 60FPS all the time (even in asteroid field). Since the gamma, I have some FPS drop during supercruise, but its pretty random.
So the 290 should be enough.
There is not so much difference between the 290x and the 970 unless the power consumption, the noise and maybe some % of perf.. (970 little bit less performant than the 290x, but its very small so it doesn't really matter).