I don't think you can get a really strong cpu and graphics card for $500.
I suggest you start with a build that uses a strong cpu and depends on integrated graphics.
Something like a i5-4440 quad and a B85 based motherboard.
Use a stronger than needed 500-650w psu that can handle any future graphics card.
That will let you play non action games using integrated graphics.
When you have the $250 or so for a good graphics card like a GTX760 you can just drop it in.