Even one 980 is probably overkill for a 1080 display. Check reviews, this is mentioned again and again
by numerous site reviewers, including here on toms.
However, if you're the type of person who likes to have really high minimum frame rates with
all details maxed out (that's kinda what I do), then I can understand the 2x 980 idea, though
don't use three as the stuttering with more than 2 cards is too annoying.
Ian.