go to nanafriend,
they're a computer business that you'll get the best pc for your money.
what they do is work with you to find out exactly what you want the computer for (gaming, in your case) and they will build a computer pinpointing things to make it best in gaming, ideal for you.
and they go by your budget too.
if you buy ANY computer from a store, it will
1.) be overpriced for what it cost to make, probably about 1/3 of your money would go to profitting the manufacturer.
2.) not be that great of quality. computers sold usually are for "general use" and all around 'decent'.
if you go to nanafriend, they build one custom for you (you can even help with the building if you want, by choosing cases and all that, they're really cooperative)
and they will make the best computer you can get for gaming for your money.
basically the only thing you pay them for is to buy the parts to build the pc.
i bought a pc from them a while ago and am very satisfied with it.
i don't think you'd get an amazing gaming pc for 500 dollars, but you'll definitely get the best gaming pc you can buy for 500 dollars, if you go to nanafriend.
all of their parts are brand new, they provide a warranty and even a lifetime of support service.
you'll like them
their page is
www.myspace.com/nanafriendpcs
hope i helped, good luck!