How much more powerful does my PC need to be to play at QHD, 1440p?
It depends on the game. Have a look at Hardware Unboxed's youtube channel and gamers nexus for extensive benchmarking.
On a 1080ti wolfenstein or some other low demanding game will net very high fps and possibly never dip below 100.
Assassin Odyssey as stated dips down to 58 fps (but its rare).
You really dont want any game getting below 60 for too long a period. If you have a lower GPU then youd need to lower settings on high end games. Some settings can alter fps quite drastically without being too noticeable.
Step up from 1080p to 1440p is a fair amount:
1080p 1920 x 1080 = 2,073,600 pixels
1440p 16:9 2560 x 1440 = 3.686,400
1440p 21:9 3440×1440 = 4,953,600
4k = 3840 x 2160 = 8,294,400 (why its ludicrous)
The thing with 1440p is that its the min resolution you need for large monitors. Otherwise you see individual pixels. Using a large monitor is sooo much better than a sub 30" - for both work and play. Id suggest a 2nd hand gtx 1080 minimum to do 1440p justice.
Keep in mind im talking about 16:9 1440p. Recommend it over 21:9. More forgiving, more compatible with other media and work and 32" same DPI as 24" 1080p (defacto font size standard) so 0 scaling issues. Also a 32" 16:9 is larger screen area than a 34" 21:9.
As said games get ever demanding. If a GPU struggles with games at a given res now its going to be worse with newer games. Who knows what Cyberpunk will really require. The best GPU on the planet cant even do 4k justice, let alone games of tomorrow. (granted with that high res you can turn off AA and that reclaims a decent amount of fps).
Regarding CPUs, if its fine on 1080p it will generally be ok at higher res. GPU power will be the bottleneck, not CPU. It helps if you have intel. There is a fair margin between ryzen and intel. Ryzen better at certain rendering and other workloads but really unless youre doing that for work, for most of us, our priority is gaming performance. ( we can wait a little longer for that movie to re-encode)