Keep directory from haveing too much

If I wanted to have a cap on the amount of space used in a directory, how could I specify to a script that if a directory is full at what level, and then move out the oldest files until within a specified paramater?

You can call du with appropriate switchs from a crontab script and take action based on your requirements to move files. Try looking at the du command and write a crontab script to check every 'few' minutes.