The SM7B is certainly a classic microphone, and I do enjoy its sound signature for vocals, but with Shure mics I'd probably reach for their KSM8 with its dual diaphragm instead. Being able to have use a dynamic without worrying about the proximity effect is just one less thing to think about for a streamer. And IMO it's more attractive than the SM7B, which will probably appeal to some streamers as well.
For $400 I'd most likely grab the RE320 over the SM7B and put the remaining $100 towards something else. Easier to drive, a tad more linear tonality, less proximity effect and cheaper. At $500, between the RE20 and KSM8, now that's a tough call.
Of course, your average gamer doesn't have the other gear necessary for these mics, so something like a Snowball or the humble BoomPro would be plenty for most.
For sure, just regular ol' gaming? A boompro or lapel mic is plenty.