I'd say overall the 750 Ti is about 10 to 15 percent faster than the 250 GTS. You have to remember, with NVidia the last 2 digits indicate the performance level within the family. However, once you get into the 50, and below, there is usually very little performance difference between generations.
That said, an NVidia 760 would be a much better upgrade. It would still need to be tweaked with AH3, but it would run much better than that 250 GTS.