Do you already have a AM3+ motherboard and DDR3 RAM? If so, then you could probably overclock the FX6300 to at least 4.1GHZ, possibly higher depending on the chip. This assumes you have a decent aftermarket cooler, because the stock one is not very good at all. Also, to achieve a decent overclock you will need a decent motherboard and good case cooling to keep the VRMs cool.
If you don't already have the motherboard, RAM and cooler, it would not be a good investment to build using 6+ year old technology (in my opinion). I have the FX6300 for my everyday computer and it works fine for my purpose (non-gaming rig), and I will keep it until something dies, then I will go to Ryzen (or whatever is the latest technology at the...