Hi zShino.
If you're only getting it for casual gaming, an i3 should be
enough. And i3 sandy bridge has onboard graphics.
MOBO + CPU.=Gigabyte H61M-D2 and i3 2100. Do make sure that the
board is B3 stepping. Price = $253
Graphics card : Sapphire/XFX HD 6450. I could play L4D2 at my
max settings with my 5450 so 6450 shouldn't have a problem. Price =
$78
RAM : Kingston DDR3 1333MHz 2x2GB. Price = $56.( 2x2GB is better
than 1x4GB as your board and cpu supports Dual Channel Memory.)
HDD : WD 500GB Blue. Price = $55
Case : Any Cooler Master case will fit everything inside.Price
range should be $60~$80. Eg. Cooler Master Elite
330/332/333/334/335//341/342/343
PSU : CoolerMaster Power Plus 460W. Price = $61
Total price : $561. It's well within budget. You can the extra
budge to get a better graphics card and PSU.