Either go with cyberpowerpc ( http://www.cyberpowerpc.com/
) or youtube some tutorials to build a $500 gaming pc -->( http://www.youtube.com/watch?v=5n5dolXyyg0
A general $500 pc will not be able to do much. It will be able to play minecraft with low graphics without mods perhaps. You will get much more performance out of a pc you build and be able to play with mods without worrying about lag with high frame rate.