Your already GPU bound with the GTX 750 Ti in almost any scenario. You probably wouldn't see much improvement with the GTX 750 Ti going to an i5-4690k. There will be CPU intensive spots in games where that would be a benefit, but for the most part you wont' notice much difference. A new GPU as DonkeyOatie said would be the most immediate benefit, but an X4 860K is so weak compared to the i5 it would be hard for me to decide for the long term. I'd probably keep saving until I could replace it all.