I wonder why do you think of upgrading while having the 2nd best single card in nvidia's which is the gtx 780. However, as an answer for your question, the titan is slightly better than the 780 even at very high resolutions
http://www.anandtech.com/bench/product/827?vs=764 check this out, this shows how the titan is SLIGHTLY better than the gtx 780 maybe by an average of 7 fps in all games. Its not worth at all to upgrade, but if you have money and want to spend it on a new gpu, then gtx 780 sli will blow the titan at lowered price, but in that case, you will need to make sure that your psu and cpu are suitable for that.