How long will a single GTX 770/780 last me on 1080p?

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GTX 770 will give you ultra settings at 1080P.
Crysis 3 is the most demanding game at the moment, the GTX 770 will outperform the GTX 680 in this chart:
http://www.tomshardware.com/reviews/crysis-3-performance-benchmark-gaming,3451-6.html

It will be a number of years before anything is released that would require you to go down to medium settings. We don't really know what upcoming requirements will be, but I expect this to be at least 2 years as games are generally developed for the hardware available when they are released.
GTX 770 will give you ultra settings at 1080P.
Crysis 3 is the most demanding game at the moment, the GTX 770 will outperform the GTX 680 in this chart:
http://www.tomshardware.com/reviews/crysis-3-performance-benchmark-gaming,3451-6.html

It will be a number of years before anything is released that would require you to go down to medium settings. We don't really know what upcoming requirements will be, but I expect this to be at least 2 years as games are generally developed for the hardware available when they are released.
 
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