Ripjaws X Series 1x8GB DDR3-1333
Price: $44.98 at NewEgg
RAM is your computer's short-term memory. It's essential for running any program and it's what the computer uses to store the processes that a program needs to access while it's running. RAM is also what lets you know your computer's multitasking capacity.
With the 8GB stick of RAM that I have recommended, you won't be able to run a lot of stuff in the background while playing games (in fact, I recommend closing most non-essential programs while running a game), but it will be more than enough to run any game that you want. It definitely isn't the fastest RAM on the market, but it is more than enough to run what you want at such a low price point.