It depends. I'd say that a RTX 3070 is plenty for 1080p /ultra preset (60+ framerates in 99% of games), RTX 3060 TI also performs well.
I don't expect Nvidia's next gen to have improved cost per frame, but the premium 4080s/4090s should be a significant improvement in performance, likely 40-50% respectively. But, it seems highly likely that they will use more power and more extensive cooling. I think we will basically be getting a scaled up Ampere on 5nm EUV.
If you are playing at resolutions above 1080p, probably worth waiting. I think RDNA3 could be quite competitive, hopefully with improved yields.