I've just done an upgrade (to
this is anyone cares) too so this is that one 6 month window every 5 years where I actually know about this stuff.
AMD cards tend to be better value for money (just in terms of FPS per £ like) and are the better choice if you're
100% certain that you'll never want to use Linux ever.
That particular card is the revised edition of the previous-generation's hardware. It's not the new architecture like the rest of the R9 family, it's essentially a reworked 7870. It's still going to be way faster than what you have now but If you can nudge it up to a 280 or 285 that'd be worth it since they are both the new architecture.
AMD CPUs are generally not a good choice for gaming. Most games, just due to the kind of stuff they do, don't make much use of multiple cores and Intel chips just have *much* better single core performance. DX12 will change that a *bit* but not enough to really matter. So if gaming is your primary concern you'd probably be better off with a slower-clocked Intel with half the cores, it'll actually perform better.
For stuff that *does* make use of multiple cores (compiling, encoding stuff) AMD chips are great because they just load them up with loads of cores/threads.
There's no benefit in getting the 1866 RAM over 1600 RAM. There's a trade off between speed and latency and after a certain point faster speeds make no difference (benchmarks demonstrate this really clearly). Having said that there's like £2 difference between the 1600 and 1866 so it's not a huge deal. I assume you're getting 1x4gb (and thus not getting the benefits of dual channel) with a view to getting another 4gb stick later?