well, for a $500 PC with monitor you'd probably want to lop it down to 350 right off the bat to account for the monitor. a decent enough 20-22" can easily be had for $150.
if you go ahead and get some basics out of the way, you can get say, a decent antec case and psu for $80, optical drive and a 500gb hdd for about 75, 4gb of memory for about 40. that leaves about 150 for mobo and proc, which can get something basic but fairly capable like an e5200 and a mobo with integrated video. basically, look at the above e5200 build above but ditch the video card and switch mobos.
if this is just a basic rig for office work or internet that would be plenty. if you want to game, you'd probably need that additional $150 to bump up the processor a little and add a dedicated video card.
