Upgrade your video card, add a SSD. Those two pieces will improve the PC greatly. What's the wattage on your PSU? Most likely, as it was a prebuilt you will need to upgrade your power supply as well.
Video card wise you could get away with an upgrade to a GTX 960, or an R9 285/ R9 390. Your CPU is weak, but it can handle upper-midrange cards.
SSD you would benefit from at minimum a 120GB drive for OS, Swap file, and your videogame you're play the most at the moment. It's easier to use 240GB+ because you can hold several games on there.
Power supply, go with any fully modular 500W+ PSU. You need fully modular if you aren't getting a new case, because having mucked around in those before, there isn't a lot of room or cable management...