Year on year gaming evolves. Sounds obvious enough, but this means buying tech now to try and stay up to date for years to come is nearly impossible. One thing which you may or may not know is that SLI doesn't work as you'd expect; its not a case of double the card, double the performance. In fact, SLI is only a 15-35% boost in framerate. So no I would not add another 760.
The 760 is a DX11 card and should be fine for the next year, maybe 2. But then when things start to get taxing for it, use the money you've saved not running SLI to get a GTX 860 or 960 (if thats what they'll be called). I recently got a 770 for £250, the same price as a 670 was at the time. But the 770 is a boosted 680 card so its a big step ahead of the 670 that it replaces. So a GTX 860 could have the same performance of a GTX 770, and a 960 could be the equivalent of a 780. Yes, a 780!!!
So wait. See what happens with new technologies arriving all the time and only when you start to struggle should you go for the upgrade. Hope this helps
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