Something like this is probably the best you can get for around $650. If you don't want an overclockable CPU you can cut something like $80 off that by getting a cheap MOBO ($50) and a cheaper i5 ($200 vs. $240.)
http://pcpartpicker.com/p/MyQCWZ
I am also assuming you are getting a good deal on a Windows key, through something like Reddit or otherwise. (Nobody pays $100 for windows.) Also $30 or so for a decent mouse + keyboard.