Start here:
http://www.tomshardware.com/reviews/pc-gaming-overclock-build-a-pc,3443.html
At this price an AMD CPU/Motherboard MIGHT give the best value.
AMD has supposedly just released price cuts for many CPU's but these prices might take a few days or so to be reflected online.
The hard part is:
a) deciding on the optimal CPU (see online benchmarks for several games)
b) balancing the parts (CPU vs GPU)
*For a "gaming build", $500 (after OS) is very, very difficult. An extra $100 makes a very big difference at this point.
I don't have time to make a build, but some quick points:
1) investigate and compare the FX-6300 ($120 here):
http://us.ncix.com/products/?sku=76934&vpn=FD6300WMHKBOX&manufacture=AMD&promoid=1304
2) Windows 8 64-bit OEM (superior to Windows 7)
*get START8 as well
(Note that Windows 8 has some AMD CPU updates now that have improved performance in some cases).
3) motherboard (AM3+)
http://us.ncix.com/products/?sku=62545&vpn=GA-970A-D3&manufacture=Gigabyte&promoid=1235
4) DDR3 (could save and get 4GB, but for $56 I recommend 8GB)
http://us.ncix.com/products/?sku=74758&vpn=PV38G160C9K&manufacture=Patriot
5) **GRAPHICS** MUST READ!!
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131472
While this is not in your current budget, it's important to note that it also comes with GAMES that a cheaper HD7770 doesn't have. The HD7850's have:
- FARCRY3 Blood Dragon
- BIOSHOCK Infinite
- Tombraider (new)
*So if these games interest you, I strongly advise throwing them into your overall budget calculations. (Bioshock Infinite in particular is an awesome game.)
Other points:
- $20, quality CPU cooler? (stock one is likely fairly loud)
- read REVIEWS on all parts
- Power Supply (reviews, ensure proper CPU and Graphics Card connectors)
- *Look at Mail In Rebate DATES, Shipping Costs, Other stores (especially if Free Shipping) and SALE PRICES.