Either of these options will improve your performance significantly. The i7-2600K gives you the Intel HD 3000 on-board graphics, and both of these cards outperform the functionalities of the Intel HD 3000 pretty significantly. As far as rendering is concerned, I had to do a bit of research, but for your purposes either of these units would improve your performance quite a bit.
Prior to Cycles Render, the GPU didn't really matter, it was all about having a beefy processor. Now, that Cycles puts the GPU to good use, buying a GPU for Cycles Rendering is a fantastic idea.
Now, as far as which GPU of those two to purchase, what you really want is a good chunk of graphics memory. Out of the two you listed, the GTX 950 would be my choice, although you get the same amount of memory, it is a newer card, and it brings with it all the features of a newer card.
From what I've read, 2GB seems to be more than sufficient for most people. That being said, a wise recommendation would be to download GPU-Z, render a scene, and see how much VRAM it takes to render. This will give you a better approximation of exactly what you need.
All in all, yes a GPU will increase your performance significantly, and the more VRAM you have, the better your performance when it comes to rendering with Cycles. CUDA is the preferred over OpenCL, making Nvidia cards the best choice. Of the two you listed, I would go with the GTX 950, which has a performance edge over the GTX 750 TI. If you're looking to save some money though, the 750 will still give you a significant boost in render speed.
Some sources: GPU Rendering FAQ |
Blender GPU FAQ |
GTX 950 Specs |
GTX 750 Ti Specs