$600 budget for computer…best possible for this is?

Given an absolute budget of $600 (which must include taxes and shipping), what is the /absolute best/ machine possible?

Note that this is not an OS-specific question. I’m asking for the absolute best including OS and enough software to make the machine useful for business and/or light-medium gaming.

Also: Assume user has no need (who really does?) for a windowed case and lighting. This should lower costs and improve price/performance ratio considerably, although it may make the machine itself look “dull” to the teenage crowd.