If you plan to keep the motherboard I would recommend a i7-6700 used if possible. Wait till later this year when Intel launches cheaper motherboards for the I5-8400. Then consider that.
Get the 1700 over the X model.
They OC the same.
Use the money on higher speed ram and try for 8 to 16GB. You will want around 3000mhz.
Also do you have RX 570 available in your area if not try the GTX 1060 3gb.
Note that the BitFenix Nova case does not have greatest airflow.
Do plan your build for tasks other than just gaming? If so Ryzen R5-1600 would be excellent choice for Streaming and rendering work also. It you want Extremely high FPS (144hz) and above intel i7 would be better.
For the GPU a GTX 1070 is ideal for 1440p but with current prices the 1070 is close to the GTX 1080 prices. The 1080 maybe the better buy depending on where you plan to buy.
The issue with 3000MHZ vs 2666 MHZ RAM was mainly due to the motherboards bios not reaching the advertised speeds. Now with most bios have reached 2933MHZ and up.
Ryzen benefits from faster ram a lot. 3000MHZ to 3200MHZ is the current sweet spot.