2 things, do you need a monitor, do you need an OS, do you need a keyboard, do you need a mouse?
assuming that you DO need those things, we're looking at about 650 dollars to work with for the build itself. maybe 675.
in that case, you basically must do an AMD build. they are the leading budget performers.http://pcpartpicker.com/p/WwXH
this is what'd I'd do on 675 USD