Your graphics card is... umm.
The HD 7750 is the slowest gaming card from AMD and the GTX 650 is the slowest nvidia gaming card. (From this generation)
These are entry level gaming graphics cards.
Your card is 10 times slower than them.
Even if you put another one in, it wouldn't make much of a difference.
If you're going to overclockm don't increase the voltage.
Keep the power consumption low as your power supply is not known for its high quality and you don't want it to fail.
Overclock by increasing the multiplier. Multiplier * reference clock = clock speed. At stock, ref clock = 200mhz and your multiplier is set to 17.5.
Set it to 20 and your CPU will be running at 4GHZ.
Also, since your case has poor ventilation, don't overclock as your CPU and motherboard might overheat.
But if it's gaming performance you're after, your CPU is not the thing holding you back.
It's pretty much the best component in your system.
If you want to be able to play games smoothly you'll need to have a decent graphics card.
The HD 7750 is an entry level gaming graphicscard. It can run every game out there, but not always smoothly even at low settings.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150612&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
If you can spend a bit more however, a GTX 650 Ti Boost will give you a much better gaming experiance. It's around twice faster than the HD 7750.
http://www.amazon.com/dp/B00CBQNFJE/?tag=pcpapi-20