"It depends".
You can set up situations where either will bottleneck the other - e.g. 4K gaming with high levels of AA, you can bring the Titan X down below 30fps in most titles, which is far less than the i5 is capable of, causing the GPU to be the bottleneck. Alternatively, you could run 1080P @ 120hz and the Titan will almost certainly not hit the framerates it could have with a faster CPU.
If spending $1000+ on a video card, I wouldn't recommend a sub-$200 CPU, but the i5 is no slouch. I think an i7 4790K or (better) an i7 6700K would be a better match for that GPU.