Well I'd say the next upgrade is a 1060 or 1080. If moneys no object for you then perhaps a 2080. But I've still got a 660ti in my setup and that runs almost anything I throw at it, so I don't think you really need a new GPU. If you performance isn't up to your expectations, it could just be down to CPU or something else, so maybe check what CPU you have and perhaps upgrade that instead? That's what I did anyway.