First and foremost, it's important to have a good balance of components. If you pair an entry-level processor with a top-of-the-range graphics card, you won't be getting the maximum benefit out of your build.
Both the processor and graphics card are important for gaming, but the graphics card plays the bigger role. Certain games are optimised to utilise processing power, but this is only noticeable when comparing a dual-core to a quad-core. As you already have an i5, this is irrelevent in your case.
If you're looking to upgrade, you'll see a much greater benefit from a new graphics card.