Zircoben :
It will, in the end, differ from game to game. Different games load the CPU in very different ways. If you see very little to no improvement in a game while using the 750 Ti versus before, you will know the CPU is bottlenecking.
However, I expect a lot of games will be just fine. What are you playing?
The games I currently play are: Skyrim, Elder Scrolls Online, Arma 3, and Battlefield 4 mainly at the moment. I will try it like you said, and let you know the results. My card will be in either today or tomorrow. Some of these games I'm sure will run fine, but I couldn't ultra most of them, and that's what I'll try with my new card.