I have the same cpu I5 8400 and i have a rtx 2070 which is close to a 1080ti when OC to paired it up with. I hit 96-97% Gpu load when gaming with no issues so you will be fine with a 980.
Ryzen 5 2600 and an amd 570 is the best for that price range imo. But what the spec of the pc that you gonna upgrade ? You might need a new motherboard and ram.
In order to hit 144hz on 1440p for latest triple AAA game the you should get a 1080ti or 2080(High-Ultra setting). 1080 will do fine with some tweak though if you can't afford the other two (Low-Medium).
Well in my country the 1660ti is selling like hot cake. Summer is near so demand for pc is insane. One of my dad friend own a computer shop so i know. The 1060 6gb is still fairly popular though since you can get them used for cheap.
Yes it will but if you can get a 1080 it will be much better at handling new game at ultra. For High setting( they look the same) 1070 will do fine in 1080p 144 fps.
In some asian country including mine the price of 2060 and 2070 is really close. by paying $50 more i can get a 2070 instead of a 2060 in my country which is why i got my card.
The 2070 is better then the 1080 when oc. The 2070 is not a weak card and it won't bottleneck your cpu. even a 2060 like xg32 said a 2060 will be fine too. But since you want a card for the future to come then go for 2080 for sure.
1080ti will smack the 1660ti to hell and back. 1080ti is a good choice or if you care about rtx then 2080 should be good. Also what is your rig spec ? If you stream or record video of gameplay then go rtx.