It is normal to have one 4gb ram stick on a store bought pc. You can add more, but I would only add 1 more. RAM works best in pairs, and you really don't need more than 8gb of RAM. How upgradeable it is depends on the motherboard in the PC. You might be able to upgrade to an i5 3570k or i7 3770k, as the g2120 cpu is an ivy bridge which uses the same socket. But a bios upgrade may be necessary for it to work, if it will at all. Additionally you should be able to upgrade or add a graphics card, but you need to make sure the power supply can handle it, or upgrade the power supply as well.
You probably won't be able to overclock it. OEM systems (store bought) generally don't have overclocking options in the bios.