yes we use NICs, if a motherboard doesn't have gigabit on board, we add a
gigabit NIC or two
all devices are connected via unmanaged
gigabit switches - you see, everything is copper, no fibre, no optical, but make sure your cables and connectors are high quality.
the plug for this type of connection is RJ45, it is downward compatible with 100 and 10 Mbit devices (also the switches), routers do only connect the office network to the internet.
everything is using TCP/IP, which i'd recommend to configure manually to avoid surprise with *selfconfiguring abilities*.
a fileserver doesn't need to be a very powerfull machine, it has just to offer a lot of channels to attach storage devices.
christian