since you don't have the money for a GPU upgrade, the difference CPU wise is daytime and night, but I would NOT upgrade simply due to your rest of components cant be upgraded. The 750ti is okay for modern games.
The reason why I wouldn't upgrade: It is not worth it to upgrade to a haswell refresh CPU. Skylake is already out and moving to that isn't bad idea. also it is not very budget friendly to upgrade GPU later and be stuck with a bad gpu with a good cpu. that's why I would save up for a completely new rig for 600-700. If you had that much, it would be better in the long term for you, instead of slowly upgrading every single part. in the new rig you can reuse the case and HDD.