The Asus Mars II GTX 580 is the fastest single video card you can buy.
By combining two full GTX 580 chips onto one PCB, it easily surpasses the GTX 590, based on cut down 580 chips, and the 6990, which is based on slower 6970 chips.
http://www.guru3d.com/article/asus-mars-ii-review/http://www.techpowerup.com/reviews/ASUS/Mars_II/
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http://www.techpowerup.com/reviews/ASUS/Mars_II/27.html