It used to be in many cases resolve wouldn’t even run on a system unless it had a dedicated gpu. Particularly on the windows side. Now with v9 this is greatly improved. So you can get away with a single card given reduced performance.
As to the 590. It would probably be more power and less performance than a single 580. It’s look at the 680 4gb or 570 2.5gb if the 580 is cost prohibitive.
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