The real answer's a lot more difficult here, as there's no hard-and-fast measure.
Simply put, it's 100% application specific; each game can place a different load on each machine's parts. Some games are much heavier on the GPU than the CPU, and others vice-versa. So it'd be useful to know what applications (games) you'd like to run are.
While it's often-repeated truth that the Core i5s offer much better gaming performance than AMD FX CPUs, citing that as an answer is shortsighted twofold: for one, the asker may already have an AMD motherboard, so once you throw in the cost of a brand-new Intel motherboard, the benefit may not be worth the purchase price. Secondly, the benefit may not be that great at all, depending upon the...