On a 1080p monitor with max or close-to-max settings, what setup will give better performance? A dual-card GTX 580 SLI configuration, or a single GTX 680? Price, power, etc. are not important here. I am only looking at performance.
Check the review for GTX 680http://www.tomshardware.com/reviews/geforce-gtx-680-review-benchmark,3161-9.html
I would go for 680 if price, money, and power is not an issue.
If you want better performance then add another 680. For 1080P 680 would suffice for most games at high settings.