Gaming (mostly) depends on the graphic card MUCH MUCH more than the CPU...
I saved this link when I was putting together my last PC>>
GPU vs. CPU Upgrade: Extensive Tests : Test Subjects: Four Generations of Nvidia Chips
You can see how the CPUs make very little difference in performance....especially if you are running graphic intensive, high resolution games
*The test is a few yrs old, but the idea is the same...unless you are going with a very very high end video card, the CPU doesn't make a big difference in performance
problem with most pre-built desktop you find in bigbox store is that they all have low-mid range video card (even if you buy the expensive top of the line models, it will come w/ the fastest CPU, 12GB ram, SSD + HD..etc etc, but only a mid range video card)