Note that there also is a performance difference if you are capping the alternate instances to a certain FPS limit. This limiting can make a big difference on what hardware you need

How many pixels do you want to render? (How many monitors? How big will each instance be?)
What kind of FPS is considered playable to you? (Varies from person to person)

I agree with others that we should list hardware specs based on FPS desired. Maybe benchmarks in a given area (Shat maybe?)