This system is quite allright as it is, balanced. Do you intend to perform a platform upgrade or just individual components? If the latter, here is the list of possible upgrades:
- CPU upgrade to i7 7700 or i7 7700k (note that your mobo does not allow overclocking)
- SSD for a system drive (250GB+)
- RAM upgrade to 16GB
If you are aiming for absolute top gaming performance, you would have to get the new Z370 mobo and i7 8700k, 16GB of RAM, SSD, and 1080Ti graphics card.
If you play at 1080p, your current card is perfectly fine for high/ultra details. Also note that 7th gen i7 is also very competitive with 8th gen in terms of gaming and will remain so for quite some time.
PSU does not need any upgrading, it is high quality and...