Not so much hardware, but some software gotchas:
- OBS's audio Monitor drifts horribly out of sync, depending on what device you connect it to. The reason is that whoever wrote that part, treated it like a network broadcast instead of the local real-time thing that it is. Every hiccup expands the buffer, ad-infinitum.
- You can work around that by connecting it to a virtual device, and then from there to where you actually want it. This works because the virtual device runs at *exactly* the same rate that OBS does, so no hiccups there, and something else has its own opportunity to get it right.
(some physical devices get their clock from the same place that OBS does, so they're okay for the same reason that the virtual device is, but some have their own independent clock which is slightly different, so they'll drift)
- Or if you must connect it directly, and you have this problem, you can periodically disrupt the flow of audio to the Monitor, which resets the buffer. Mute/unmute, change the Monitor setting, anything. The stream and recording don't have this problem, so no need to disrupt those if you can avoid it.
- OBS's audio in general is not all that great. It sounds fine, and it does have a few processing tools, but it's nowhere near what a sound studio would have. It sounds like your rig might be okay with just OBS, but do keep in mind that if you want to do much of anything beyond that, you may have to give up on OBS's audio altogether and move all of it into an external tool instead, whether physical or virtual. Then OBS only has one audio source at all, as a "dumb passthrough" from that external tool.
Since you've separated your gaming and streaming rigs (good idea!), I'd suggest an internal HDMI capture card with passthrough. The gaming rig feeds that, which then feeds the gaming monitor. Or you can use an HDMI splitter; the capture card's passthrough is essentially that anyway.
Gaming sound also goes through that HDMI cord, so the capture card naturally picks that up too. If that creates a challenge to be able to hear it yourself, then that's a problem worth solving, but not by passing it through the streaming rig. HDMI speakers in some form or another, or an HDMI audio extractor. So no matter how bad the stream gets, you can still salvage the gameplay.
In fact, you might not even realize that it's acting up until you watch it back later. You probably want that if you don't have a separate Broadcast Engineer - just focus on the game - but if you do, then of course they'd want to hear the stream as it happens and fix whatever problems they find in real-time. OBS's audio Monitor would only be used for the Broadcast Engineer, so if you don't have one, then it becomes irrelevant except for a quick check to start with.
That pretty well covers the game audio...except for a conversation with other players. That will always be tricky unless you're okay to keep everything that you hear yourself. Otherwise, you need to put on your Audio Engineering hat and chase the signal routing in detail through the game and related things.
For a mic, I'd keep the stream comments and conversation with other players separate. Two mics, each wired to a different box, and possibly with a different mute switch for each. Depending on your game settings, having both on may or may not echo, so keep that in mind.
Having two mics may seem like overkill at first, but it guarantees that you don't have electrical problems from connecting the same one to two different boxes, or routing headaches to try and get it from one box to another without getting everything else along with it, etc.
