-4GB is pretty solid, so i wouldn't upgrade that. for mid range gaming you wont see a difference
-power supply by itself will give you 0% improvement. The reason to upgrade your power supply is so that you can upgrade other things such as video card.
-Video card would be your best/cheapest option if you wanted to get into mid range gaming. You wont be able to get anything too crazy with such a "small" power supply but you should be able to upgrade to something.
Kinda light on time, cant go too detailed, but i hope that helps.