680 sli is about the same as a 690.
https://www.youtube.com/watch?v=Q9SsqQv8JrI
and according to this the 980 is usually better than a 690 and the 970 is probably about the same and you dont have to deal with SLI
https://www.youtube.com/watch?v=X9wsd_T7fV0
i think you should either get a gtx 970
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121899&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-PCPartPicker,%20LLC-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
or an r9 290x for a cheaper option that is as powerful as a 970 but is also less efficient and toasty hot
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150696&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-PCPartPicker,%20LLC-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
or an r9 390 for 1440p or 4k
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121962&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-PCPartPicker,%20LLC-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=