$500 is not much, although prices in the US seem to be a little lower than in Europe, guess it's because of the VAT. Anyway, for $500 you can't expect a high end rig. Some brand computer beefed with a decent graphics card might be a good choice, look for Brookes comments about Dell in the links I posted. Another choice might be upgrading your existing rig. Chances are, the only reusable parts would be the case and optical drive, maybe not even those. Or, depending on what you've got, you'd "only" need a motherboard, a CPU, RAM and GPU to start with. So, if you're able to reuse your current rig, tell us its specs.