I'm kind of wondering where you came up with a "K" in the model number of that CPU...
I couldn't find one, only the G-3258.
I know this is a bit off topic, but the new Skylake i3 CPUs hold their own against many of the i5 quads, with respect to FPS
(*). Plus, the new X-170 chipsets allow you to use staggeringly fast RAM, (DDR-4). This has the overall effect of overclocking the CPU automatically, and frame rate increases can be on the order of 20% higher with faster RAM., as opposed to 2133 Mhz stock RAM, which is the max for H-xxx boards, and Intel's IGP on die graphics. You're limited ATM with the 2 thread Pentium. Any Haswell i3 should turn in much better frame rates with their pseudo 4 core architecture. It has to be noted though, the Skylake memory controller is allegedly a big improvement over Haswell's.
All of that notwithstanding, the better graphics card would likely be the more sensible way to go, since "you can take it with you", so to speak,, to your next build.
Provided of course, as Cliff has pointed out, your current PSU has the necessary oomph to support the step up video card. If your current PSU has been in for a couple of years, it likely can no longer supply its rated wattage. The capacitors get old after facing a lot of heat, and there always exists the possibility of destroying your system with an aging PSU.
(*) These new Skylake i3's have been tested with GTX-980 cards, to eliminate any and all possibility of the video card bottle necking the CPU's.