The main purpose is speed improvement as I understand the local file transfer will move from 1 x 100 to 1x 1000 is this true...
...and what are the pro's & con's of using this type of router with A fibre connection.
Unfortunately I can't be sure as you haven't given me any specific details about your connection or the specific gateway device you will be using, however there are probably none either way:
1.) A modern router is a router and a switch. In most cases only the switch part is gigabit, the WAN connection is still only 100Mbit.
2.) Even if you get a router with a gigabit WAN port, the modem (gateway) you are going to connect it to is probably 100Mbit.
3.) Even if you had a gigabit WAN port and a gigabit modem, your ISP will probably cap your speed at 100Mbit or less.
4.) Even if you had a gigabit WAN port, gigabit modem, and a true gigabit connection from your ISP, few sites you connect to will offer even 100Mbit connections, much less anything higher.
In short, it is doubtful that your internet connection will be affected in any way.
Could you also let me know what specifications to look for in this type of router?
That is a loaded question. Did you need wireless? If so, what type (a/b/g/n)? Do you really need a gigabit WAN port? How many LAN ports do you need?
If you already have a router, I'd recommend leaving it in place and simply getting a gigabit switch. Pretty much any of these will suffice: http://tinyurl.com/69tk2b
I never seem to hear anyone discuss this topic is there a reason why?
Probably because not many people have a server in their home (which is too bad, because they *should*), and gigabit is most useful when moving files to and from a server. Scott.