How to delete the oldest X number of rows?

Hi,

I have a table that logs the performance of a system. I get an entry in this
table each time the system does something.

I want to keep the last 50,000 rows in the table. I'd like to run a cron job
every 5 minutes that will delete all rows > 50,000, oldest first. What's the
most efficient way to do this query/delete?

DELETE FROM log WHERE timestamp < ????

I can come up with a way to do this using COUNT(*), but I'll bet there is a
smarter way.

THANKS for your help!

John