Your system is very similar to mine (same mobo and cpu, and I had a 7950 until about 7 months ago), so here are my experiences:
-Having upgraded to a RX480, I have found I got CPU bottlenecks at 1080p even at a higher clockspeed than you (4.4ghz). I would bear that in mind as your system is pretty well balanced currently (what resolution is your monitor?)
-I have 12GB RAM which has proved sufficient. The only reason I have 12GB rather than 8GB is because when I upgraded years ago from 4GB, for future proofing reasons I didn't want to fill all slots and only have 8GB. Given how expensive RAM is these days I wouldn't bother, a couple of years back you could get another 16GB for like £50 but not anymore (although checking just now things aren't quite as bad as they have been). I have on occasion seen usage go over 8GB but it is pretty rare.