There will be some bottlenecking with a 750ti and a core 2 quad processor, but you will still see an improvement over the 9600. You would be better off getting a new motherboard and cpu as well if you can afford it.
If you run a q6600 or better and especially if over clocked it can stand it's ground surprisingly well. I had mine run with a gtx 570 at the time so I can't see why yours wouldn't last a bit longer with a 750ti for lighter gaming needs. That processor is about 7 years old which really limits it's ability to perform in high demand newer titles though so if you can afford it in the next year the performance jump would be huge with a build with an i5 and gtx 770 for example.