With Mantle leveraging your chosen CPU, you're gonna be fine. But the thing is, support for Mantle is only picking up and there are numerous games that would be bottlenecked by your CPU. This problem could be mitigated by overclocking your CPU, but if you want to be free of any potential bottlenecks, you could cut costs on your case and power supply (850 W is too much unless you plan to CrossFire your 280) and grab a four-core i5.