Your CPU is way to weak and RAM way to slow for Fallout 4 (which for some reason requires the fastest RAM possible) so you can forget that. Realistically you're looking at the 270X as the top option, but you're still only looking at FPSs in the 30s for any modern game.
not even close, I have a a8 5500 apu which is weaker then the 860k in my youngest sons pc , it currently has a r9 290x in it " left over gpu from one of my pc upgrades " and even cpu heavy games like gta 5 plays very well with this pair , ofcource advance graphics are off , but all standard graphics are ultra with fxaa only for aa and holds a steady 60 fps with the worst dip during fast driving / shooting scene was 42 fps , I'm more then surprised of these results myself , we are waiting for the 880k launch although have been debating to get the 860k as a release date hasn't been released yet for the 880k
I do agree you need more ram that above pc I mentioned currently has 16g of 1866 adata .
sure some games will bottleneck gpu , but this can be said for all builds for x game to be honest . take a 5960x and an furyx and some joke engine game like arma and even that cpu wont load up that gpu .
as always when looking at gpu upgrades , look at the games you play and buy the gpu with the most fps in gaming benchmarks that fits in your budget .