It is a small upgrade. The main advantage of the 750ti is energy efficiency. Its barely more powerful than a 2GB 650ti boost.
My opinion is you are better off sticking with your current GPU until you can get something more powerful than the 750ti for the same price. For example the R7 360X looks like it will be a very promising budget graphics card when it arrives later this year.
People are expecting it will have as much power as a GTX 760 or 7950/R9280.