Good choice man! Actually the 970 is the card I recommend to everyone that asks because of its amazing power efficiency and great temps at a very competitive price. I mean 145w TDP is just insane, that's over 100w less than a r9 290. AND it's a badass card, overclocks like a bat outta hell too for that extra little cherry on top.
I would say you should definitely be able to hit a solid FPS in arena commander. It's hard to say right now because there's a 60fps cap and we have so little of the game to gauge, but there's no doubt in my mind that Star Citizen will be amazing.