its probably also worth figuring out what takes longer gpu or cpu.
I’m also not sure how well rc handles multiple numa nodes. but think it suffers a bit so your probably best to stay single cpu rather than dual xeon. esp if you don’t need the extra ram.
adding anymore video’s cards will only add a little bit of extra speed.
for cpu’s you could go a 6900k or 6950k. (im running at 6900k @ 4ghz).
or you could go for a single xeon like a e5 2687w v4. just keep in mind keep below 16 cores unless you want to disable hyper threading.
probably by far the cheapest thing would be to get a few machines, at similar spec to what you have.
I think going for 1 very powerful pc, makes sense if you need lots of ram, or are getting cli version.