I had a 850 for my 4090 and 13900k with no issues at stock speeds.
HOWEVER - When i started to overclock both - i found some instability with the GPU. Going to a 1kw knocked out any stability - and as a bonus, some coil whine dropped also.
You should be fine in other words for the setup you are thinking!
You will surely see a nice bump in performance across the board at that resolution. Albeit, many of the newer games would benefit from the extra cores of a i7 - so keep that in mind. Why? depends. If you are running RGB software, Fan Software, Streaming / Chat Software, External Sound Device, etc. etc - all adds overhead. With the most recent titles like Hogwarts, Starfield, even Battlefield 2042 - the use of more cores is becoming more prevalent. The worst thing is to have a CPU bottleneck. Unlike a GPU bottleneck, a CPU bottleneck manifests in dropped frames - ie stuttering. That for me is worse than low average FPS. I rather have a low average FPS than stuttering (ie low 1% FPS). So the point is if you have a clean system, no "extra" software running, you will be fine. But if you plan on running "extra" software in the background. I would bump it to the i7 - and your not looking at much more cash.
Other point - he b760 is fine, however, dont expect to overclock. If you are OK with keeping everything at stock configuration; that includes DDR5 XMP over 6000 - dont expect it to work above that. You might even need to run at 5600 (stock DDR5 speeds). If you want to overclock go z790 period. But if you are OK with stock settings, you should be fine. Another point - make sure you are happy with the onboard devices - that also is another factor.
You should be at 35k with stock settings
Could be programs running in the background. Make sure everything is turned off including the Nzxt software