You probably won't be bottlenecked (maybe in really CPU-intensive games), but considering that your CPU is not the most powerful one out there, I wouldn't go any higher than R9 270X. It's on the same level with the GTX 760 which I have and trust me, it's worth getting if you've got the money.
Whether you're going for the R9 270X or the GTX 760 is completely up to personal preference and local prices. http://gpuboss.com/gpus/Radeon-R9-270X-vs-GeForce-GTX-760
According to this the GTX 760 performs a tad better but it costs a little bit more, at least if you're from the US.