Anyone here could show me any documentation which could help me justify the use of gigbit copper NIC on servers instead of fiber?
In my experience servers typically doesn't gain much from having a fiber connection instead of 1000MB copper, but if you ever found a document showing a quantitative approach to justify this, please let me know.
Not sure what environment you've got, but surely price is an almost universal justification.
Fiber NIC's are few and far between whilst most servers can be purchased with a 10/100/1000 Mb copper interface. Fiber patches and interfaces to switch infrastructure cost more and are less dense than their copper equivelent.
Only justification I have for fibre terminations is Security (see paranoid, defence type installs), electrically noisy environments and distance requirements.
IMO should be pretty easy to justify just on cost benefits alone. What issues are you dealing with that make it hard?