Well, I don't own it, but I do own an FX-8320. The problems with the FX-4100 are twofold:
1. It's from the earlier Bulldozer/Zambezi line, which were not quite as optimized as the later Piledriver/Visheru chips. Not that they were "bad", per se, but in some ways they didn't perform as well as the Phenom II chips they were replacing (http://www.anandtech.com/bench/product/362?vs=1280).
2. It's the lowest chip on the FX totem pole, & we're starting to see games come out that need at least the FX-6300 or FX-8300, which means games are starting to take advantage of the additional cores in FX chips. The problem is, your chip only has so many cores, & as @Ecky pointed out, its performance ends up being somewhat lacking because of that.
Like @Ecky said, your options are going to be limited by your budget. However, there are some things you can do that won't be too invasive. For example, your board is limited to the 95W CPUs (http://www.gigabyte.com/support-downloads/cpu-support-popup.aspx?pid=4311), but luckily that includese the FX-8300 chip. That's still one of Tom's Hardware's recommended mid-range CPUs (http://www.tomshardware.com/reviews/best-cpus,3986.html), & it's only running $105 USD on Amazon. Replacing that will help out a bit with the CPU-intensive games, & moves you up from a 5th-tier CPU to a 2nd-tier CPU (http://www.tomshardware.com/reviews/cpu-hierarchy,4312.html).
The other thing that's going to hold you back, though, is the GPU. A GTX 570 is probably going to have some issues nowadays. I'm not sure how reliable your PSU is; however, given that its official rating is 750W, we have some room to work there. Since I'm guessing your monitor probably tops out at most at 1080p resolutions (if even that high), you don't have to worry about getting a seriously powerful GPU. For you, even a GTX 1050 (no PCIe power connectors) would be a decent upgrade. Personally, though, I'd see if you can swing at least an AMD RX 470, if not their 480 or a GTX 1060 (6GB model). They'll all pair well with an FX-8300 chip, & move you up from a 10th-tier GPU to a 6th- or 5th-tier GPU (http://www.tomshardware.com/reviews/gpu-hierarchy,4388.html). Depending on the model, they're currently running about $250-300 USD for the RX 480 & GTX 1060, but there might be some Black Friday deals you can take advantage of.