Which graphics cards

Solution
You will win FPS benchmarks with GTX970 sli.
But, at only 1080P, I think a single GTX980 will be a better experience.
dual cards may have issues such as screen tearing or stuttering because of minor differences in performance of separate cards. And... some games do not do well with dual cards.
Look for a factory overclocked GTX980. The small premium gets you a better bin and a warranted overclock for not much more.
If I recall correctly 2 GTX970s are better for 4k, but I would think that they also perform better than 980 in 1080p. Personally I would go with the 980 because it's one of the best and it leaves space for a further 980 SLI upgrade, squeezing out even more frames
 
You will win FPS benchmarks with GTX970 sli.
But, at only 1080P, I think a single GTX980 will be a better experience.
dual cards may have issues such as screen tearing or stuttering because of minor differences in performance of separate cards. And... some games do not do well with dual cards.
Look for a factory overclocked GTX980. The small premium gets you a better bin and a warranted overclock for not much more.
 
Solution

True, but just paying $100 more for 2 cards that have been shown to clock to 980 levels is better than paying $200 for one slightly better card.