Honestly, I could give you a PC build for $500, but it was be useless, as price for performance ratios would be horrible, but in the case where I recommend saving some more, maybe up to about $800 - $1500, the performance you could get from a PC at those costs will be much more worthwhile, will give a lot of performance, and will be quite future proof, for next-generation games.
An example is that, a decent mid-range economical graphics card around now will set you back $150 - $300.
An HD 7770, which is an alright budget card, will set you back $100, while another mid-range performance card, the 660 or 660 might set you back upwards of $200. This is why you must save a lot more money.
My conclusion is, you shouldn't try to build a "gaming" pc with only under $500 dollars.
Save your money and get something of higher quality!