For a single graphics card stick with the i5. If you have plans of multiple high end GPUs then I would advise you get the i7, although I would also advise you at the same time to stay away from SLI/CF lol.
The i7 tends to get slightly higher minimum frame rates. The i5 tends to get slightly higher maximum frame rates. In most games it really doesn't matter. At 1440p with max settings you will be almost 100% GPU bound anyway so get the best graphics card over the best CPU.