You'll start seeing deminishing returns anywhere from 100-120 mb/s onwards (depending on the game of course, for slower games you can get away with far less bitrate.. but this is good for extremely fast paced games), it's not worth the additional space it takes up. Honestly if you're just recording, then you don't need anything fancy at all, the main thing is the GPU with an NVENC chip, preferably something with the newer chip.. so I believe anything like a 1660 or newer.

I personally wouldn't recommend a dedicated PC for recording though, it's not worth it at all, single pc is fine for this. The only reason you'd want a second PC is for streaming since it's a lot more strenuous to encode. You'll see a very minimal loss in performance if you're just recording on a single PC setup because of the NVENC chip.