Worth is something only YOU can determine.
If you are now having a problem with graphics performance, then yes, a GTX980 will be an upgrade, but it might not be sufficient.
I am somewhat in the same situation, anticipating a 4k monitor.
I have decided to wait until I have such a monitor and then determine what I need. Perhaps by then a GTX980ti might be available.
If you need a big boost today, and your games can effectively use dual cards then GTX780 sli is a reasonable option.
I would buy the second card used,.
Remember, you will be using 225w more, so you will need about a 850w psu.