When do you stop outsourcing IT/IS functions?

I am the most tech-knowledgeable person in my company. We have a local network and approximately 20 computers. I perform basic to midrange server admin, and almost all the desktop tech support. At what point does it make financial sense to have a full time in-house IT employee? We have an outside IT company for major issues (until I was hired they were for every issue). We are currently running Server 2003 and XP on the desktops.