You should be able to do it with $500 or less. The real trick is keeping your GPU cool so get a good fan setup on your GPU. The game doesn't use much on the CPU side I got a 4 core 3.4ghz and it runs at like 30% most all the time. If you cant get a decent pc put together with $500 or less your getting ripped off.....
but i ill stop argueing now... since that guy gives ya detailed infos... non or less the biggest part have to be the gpu... and everything below 150 € is just .. plain stupid.