Case cooling isn't what's holding back your i5-4670K temperatures. The Zalman 10X Optima is far from the best CPU cooler you could put into a 300. And you've most likely not delidded your CPU, so its heat transfer ability is limited by the poor Intel TIM between the CPU and the IHS.
Unless you have a dual graphics card setup, the side fan is relatively pointless, it'll just add decibels. The two front fans are nice to have (with dust filters installed) just to balance the pressure of the case and avoid too much dust buildup.