Hey man, i'd imagine you already picked up something by now, but anyway...
With $500 to spend u should be able to find something pretty decent.
Don't pay to much attention to brands... these days most things are of equal quality.
Instead look for things like sealed bearing hubs, a full cromoly frame, fork, handlebars cranks etc... and double walled rims.
The more of these boxes you can tick off, the stronger it will be, and therefore longer before you find yourself needing to upgrade parts.