DX12, by itself is not a big deal. You will get the advantages of it when game developers take the time to implement it.
They may not do that since it will take more effort to implement low level code.
The faster path through DX12 graphics will help both types of cpus.
To some extent, 8 cores will be helpful since the FX-8 has more cores.
Still 4 faster cores are probably better. Read up on "Amdahs's law"
If you will not be overclocking, I would rank gaming cpu choices in this order:
i7-4790K 4.0/4.4 turbo at stock.
I5-4690 or i5-4690K @ 3.5 The extra hyperthreads of the i7 and xeon chips will go largely unused in games.
The value of a 4690K is that you can easily get 15% more power with a mild overclock. It is a nice option to have, even if you have no oc plans initially.
I discount the FX chips, they are old and slow for gaming. OK for multithreaded apps.
FWIW.
I will never again build without a ssd for the "C" drive. It makes everything you do much quicker.
120gb is minimum, it will hold the os and a handful of games. If you can go 240gb, you may never need a hard drive.
I would defer on the hard drive unless you need to store large files such as video's.
It is easy to add a hard drive later.
Samsung EVO is a good choice.
Intel 730 is OK too.
The value of a ssd is not so much for gaming as it is for everything else you do.
But, consider the source that says a ssd can help fps consistency:
http://www.samsung.com/global/business/semiconductor/minisite/SSD/global/html/why/forGamer.html
On graphics, amd and nvidia will have similar performance at equal price points.
The R9 cards get their speed with aggressive clocks. That adds to power requirements and some cards have problems.
Look at the proportion of negative reviews on newegg and compare to GTX970 .