Maybe read this article..http://www.tomshardware.co.uk/system-Builder-Gaming-PC,review-31760.html
..and see if you can re-use any of your existing bits to knock the cost down from $700 to your $500 budget. Also give some thought as to whether you would want to overclcok it to get the most out of what you buy. OK it's using an old socket which isn't so good, but on the other had that is one of the constraints of the tight budget. There are also some options around the graphics cards here I think, you could start with only 1 to meet budget with a view to upgrade later or go for something different entirely.
But anyway this is one way an experienced builder thought was a good way to spend $700 in December so things won't have changed a huge amount..