ALSO,
It's not clear what COMPONENTS you have already. If the CPU is okay, and the GPU so-so or weak, then you may want to simply put in a better graphics card such as a GTX1060 6GB for about $250USD.
I wouldn't rebuild the system if you have an i5-4460 or better CPU (though a weaker CPU may still be adequate depending on what games you play).
I recommend listing your:
a) CPU
b) Power Supply (if possible, or at least open up to see what the WATTAGE is such as 300W).
c) system memory (8GB in general is fine for gaming if you don't have web browsers open with demanding games)
d) Windows version (i.e. Windows 7 64-bit, W10 64-bit)
e) Graphics card (i.e. GTX660)
Use CPU-Z, Speccy, or other software to find out that information.