The fibre is used to connect the gigabit switch to your backbone (where I think you have to have a core builder). We've recently put in a gigabit switch 1.5k by the way for a 24 port. Once the switch was in and the fibre connected up I used the exisiting Rj45 cables in the server room which are at least 3 or 4 years old and connected up the servers with the gigabit cards.
now 1gb transfer between to of these servers took less than 25 secs
sweet
Baz