I dont understand why you want to compare a 3570k with the FX-8120 since you got Intel platform. But to answer your question the 3570k easily beats not just FX-8120 but also overclocked Fx-6300 since its the best gaming processor at this time. If your planing to run the 3570k non overclocked then you might aswell get a i5 3470 or non K 3570 to attain the same perfomance in games but @ less cost.
Following is a comparison : http://images.hardwarecanucks.com/image//skymtl/CPU/FX-6300-FX-4300/FX-6300-FX-4300-83.jpg
Notice how the CPU performance lineup @ stock speed with same HD7970 GFX Card:http://www.techspot.com/articles-info/608/bench/CPU_01.png