Real name as user slug in profile

I would like to change my set up so that the slug of the users is their real name.

Say I have a user who's real names "john smith"
his username is "bilbo"
his slug now becomes http://www.examplesite/members/bilbo
I want it to be something like:
http://www.examplesite/members/john-smith/

How can I achieve this?

thanx
j