I don’t know if you ever watched a show from back in teh glory days of TechTV called The Screen Savers, but they did a segment every so often about a $500 gaming PC. They’d go out to pricewatch.com and such and see what they could put together for under $500, and they’d use whatever monitor and keyboard and mouse they had already. Google $500 gaming PC for some ideas, or try to go it yourself.
And Nickname actually has a darn good looking list to start on, by the way.