extra harddisk on RAID5

[roland <roland cat be>]

Hello,

I have a server with 3 Harddisks 74Gb Raid5
I would like to install a 4th disk as a backup.
I never did this before. What will happen when I plug this disk in,

will it install itself?
how will it appear?
When one of the disks should fail in the future, can I pull out this 4th  
disk and plug it in as replacement for the failing disk?
Or there some things I have to take care of?
....

Thanks for your help

--
Roland Brouwers
C.A.T. bvba