1) As said it's best to have high DDR4 bandwidth. Preferably at least 3000MHz CL15 in dual channel (two sticks not one). Also preferably 8GB though if budget is tight get 8GB now with two sticks but make sure the MOTHERBOARD has four slots so you can add another IDENTICAL kit later for 4x4GB.
Ideally though:
2x8GB 3200MHz CL15 DDR4 (or close)
2) Many games will work fine at 2560x1440 if your monitor is higher than 1920x1080. There's always some ideal combination of game settings, resolution and FPS (and VSYNC on/off) that gives you the best experience.
3) some games will be CPU bottleneck and some GPU bottlenecked but overall an R5 + GTX1060 6GB isn't a bad combo
4) If you have any SPECIFIC questions post them too
5) OVERCLOCKING may help but DDR4 bandwidth is most important for CPU performance.