Do crons take up alot of server load and bandwidth? I have a php script that checks to see if any entries in a MySQL database are older than 5 minutes, and if they are, it deletes them. I was hopeing to possibly run this cron several times.

Cron daemon itself doen't take much resources it just immitates uou starting your script. The real load is produced by the script and it fully depends on you as script developer whether it creates much load or not.