If you aren't sure whether you need the GTX 750 Ti or a GTX 970, then you haven't really decided on what your gaming expectations are yet. What's your gaming resolution, what settings do you intend to use, and what framerates do you expect to get?
I wouldn't get too hung up over "CPU bottlenecks." Your i3 isn't going to be any faster with a shitty video card than it will be paired to a fast one. Most games will not be drastically performance-limited by a Haswell Core i3, at least not to the point where a 970 has no benefits over a 750 Ti.