I've been on hiatus recently, but I've boxed 15 characters throughout a lot of WoD with this build:

I7-3930K oc'd to 4k
2x GTX 970s
32gb RAM

When I do cap out on resources, it is invariably CPU.

I'm interested in moving to 40 boxing, continuuing into Legion. Presumably, this would be best accomplished with two machines running 20 toons. Based off my experience with the above build, here's the shorthand of the build I'm considering (http://pcpartpicker.com/p/swf8Lk):

I7-5820K oc'd to ~4kish
2x GTX 970s
32GB RAM

Is this build going to comfortably run 20 toons? Can anyone compare resource usage of Legion to WoD? I'd far prefer to just run two computers for simplicity's sake (two monitors vice three, etc), but if I must I could consider three.