For general settings to start with you can allways use the estimator:
https://obsproject.com/estimator
But in general you will probably want to start with something like this:
Base Resolution either 1080p or 720p depending on your xbox games
If 1080p set resolution downscale to 720p
FPS 30 or 60 depending on your game
Bitrate 2000-2500 kbit for twitch.tv and unpartnered streamers, you can increase it up to 3500 if really necessary
And last but not least, since its a dedicated streaming PC you can probably increase the encoding preset, thats in the advanced settings. Default is very fast but you might be able to do fast or even medium, keep an eye on your cpu usage in fast action scenes, if you get high encoding cpu usage, lower the preset from medium back to fast.