To run the full text procedures you have to be a member of the db_owner fixed database role within the database you are running the procedure.
(For SQL 2005+)
To run the sp_addlinkedserver procedure you need the ALTER ANY LINKED SERVER right.
To run the sp_addlinkedsrvlogin you need the ALTER ANY LOGIN right.
(For SQL 2000-)
To run the linked server procedures you need to be a member of the sysadmin or setupadmin fixed server roles.
For most system stored procedures if you look up the procedure name in the Books OnLine index, towards the bottom it says which rights are needed to run the procedure.