RTX 3080. Modern NVENC provides a video encode quality on-par with x264 Slow, with zero in-game impact when correctly configured.
AMD's equivalent, AMF, is... very, very bad. Poor encode quality, uses game-rendering resources for the encode, misreports feature levels (so can just crash if something tries to use a feature it says it supports, when it doesn't).
(edit) I forgot that the 5900X is a CPU, not a GPU. Derp. Still, use NVENC. There's almost no quality gain from a software x264 encode as compared to modern NVENC, even at the most taxing preset levels. NVENC is why 2PC setups are effectively dead at this point.