Even if you dont buy a tower and just build it on a slab of wood or something (not recommended

) your still up for more than $500.
Ram = ~80
HDD = ~100
CPU = ~ 150
GPU = ~ 250
Thats not including power supply, mobo, dvd drives, sound cards, etc. No budget gaming comp is possible at $500 thats pre-built, and building one yourself, you'll probably have to try and pick up some second hand things to make it fit in your budget.
Sorry to put a downer on it bud =] But it is going to be very tough to pull much from $500.
What parts are in the old one? You might be able to build a new one and use some of your old parts =D What you got?
Everything I said is almost irellevant, try and coax an extra 150 and heres your build mate =]
http://www.overclock.net/amd-general...ild-650-a.html
Its got everything you need for a pretty damn good build.