My brother has that same monitor and gets great frame rates with his RX480 (equiv to GTX1060 6GB) and i5-4690K. Since the monitor's max refresh rate is 75Hz, having a great deal more power than that may be a waste for today's games. That said, a GTX1660Ti will have a decent amount of extra power to be able to max out your monitor for the forseeable future. Make sure you enable FreeSync
and enable a frame rate cap to save power/heat/noise in less demanding games.
No need to upgrade your mobo. If you hate the onboard audio (this may require further discussion), you could consider a $30 discrete audio card.
Getting 2x8GB DDR4-3000 will be a nice bump compared to what you have. Costs about $75
Ryzen 3xxx CPUs launch in July. Wait till then to consider a CPU or mobo upgrade.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod