For gaming, the first thing to upgrade is your GPU. A lot depends on your budget, but the GT730 is a pretty terrible card for gaming. If I knew how much you could spend, I could make a more specific recommendation. Right now, there are some great prices on GTX 960s if you shop around.
After the GPU, you could get a more minor boost by upgrading your CPU to an i5 750 or 760. Those only cost $30-$40 used on ebay. That will bump you up to a quad core CPU.