Yes, one GTX 760 is significantly faster than 2 GTX 555's. + double the Vram.
So it would be a good upgrade.
However depending on your budget you may benefit more by buying a GTX 970, or if it's too expensive an r9 285. (unless you prefer Nvidia) A GTX 960 would be the newer equivalent of a GTX 760.
It's up to you, you may have found a good deal on the 760.
-good luck