1| That would depend entirely upon the apps/games you intend to play on the system.
2| Even if you upgraded your CPU, it'd be a moot point since your processor would be limited by the chipset and the ram kit(assuming). Overclocking would alleviate some/all bottlenecks but then again at 1080p gaming you're not really taxing the system much.
3| I'd be primarily concerned about the PSU's make and model and if it's capable of handling the
additional system power draw once you drop in a GTX1070/1080 in.
You'll be fine with your existing system provided the PSU is up to the task.