Gaming Computer Under $400

Hello, I am looking to get a new PC that can handle Diablo 3 and Starcraft 2 on medium settings. I am doing this because my iMac can't handle these games very well. I have heard of people building their own machines but this sounds very risky and hard to do. Am I wrong? I am also only 14 so money is very limited and I also have little experience with computer hardware. I know all of the components of a computer but that is about it. What do you think I should do?

Budget Range: 300-400
System Usage from Most to Least Important: Gaming
Are you buying a monitor: No



P.S. I don't mind refurbished items.