i suggest you sli two 970 cards... or... crossfire two 290x cards
both are almost equaly great at high res gaming and 290x is cheaper
980 is is not bang for the buck right now.. it has around 15% more performance than 970 and cost a lot more
if you have a big budget then just buy a single 295x2 ....its far better to get a single powerful card than sli or crossfire
http://www.hwcompare.com/18215/geforce-gtx-980-vs-radeon-r9-295x2/