The question about a i5 2500k got me wondering. I have an i5 2500k at 4.0 GHz and I was wondering how much performance am I losing if I were to get a GTX 950 Ti vs. paired with a new CPU.
First off, your Intel Core i5-2500K is fine - thanks to you overclocking it to 4GHz, you've got performance that most people don't when they leave it at stock. As for the upgrade, if you're playing games - then your CPU will be absolutely fine, and I would suggest you upgrade your GPU.
There isn't an NVIDIA GeForce GTX 950 Ti, but there is a GTX 950 and GTX 960. Either of these would allow for 1080p gaming at 60FPS, which is going to be great.
But, if you upgraded your CPU - you wouldn't see much performance benefits in day-to-day use, or in gaming. If you were doing CPU intensive things like video editing and number crunching work - 100% yes, the CPU upgrade is the path you want to take.