Depends on what resolution you want to play it at? Also, if your card is handling the game at the settings you want and resolution, why upgrade? You are not using all the cards resources, only about 85% of them. Though, if you want to increase resolution and settings...I HIGHLY recommend SLI 970's. They are the best performance/cost card on the market right now and SLI 970's out perform the GTX 980 on 4K with any top game at max settings. Also, the 980/970 have 4GB of VRAM just like the GTX 760 you have now. So, if you are looking to increase your VRAM getting those cards along will change nothing.
Your best bet, based on your budget, is to SLI 970's (about $600). They will provide a GREAT 4K experience on ultra with any game out there for the next 2-3 years (assuming your have a powerful PC to match).
I just got my new system, see signature, and my plan is use the single GTX 970 for now. Then upgrade to GTX 970 SLI later when the price drops and/or I see performance issues in my games. Cheaper than a single GTX 980, more powerful overall, and cheaper if I buy them overtime when the price drops for the second one.