Here's a suggestion
CPU:
pentium g2020 it's really no slouch, I had a g860 and was impressed
Motherboard:
biostar h77 nice board (see previous post), and free 8gb of 1600mhz ram
Memory: comes with board
Storage:
WD caviar blue 1 tb drive nice drive, 7200rpm
Video Card:
powercolor hd 7850 1gb it's only the 1gb model, but overclock it and you'll be fine
Case:
silverstone redline very well built, good looks, and dust filters
Power Supply:
corsair builder series 430w psu great quality, enough power
Optical Drive:
Samsung SH-224BB DVD/CD Writer if you actually need it
Total: $492.93
this build will be able to play any game at decent settings, you can save money by getting a cheaper case, or hdd, or not get a dvd drive, but those are the only things I would change with this build, unless you save some money to get the 2gb 7850 for $20 more, which is actually a worthwhile investment as you won't run into frame buffer problems, but the 1gb 7850 is still a good card
edit: psu just went out of stock, so get this
antec basiq instead, same power, just not 80 plus bronze, still good enough though