Most mid-level enterprise servers now have dual gigabit NICs -- but most of them don’t use the second pipe. Moreover, gigabit switch prices have dropped through the floor. With a 120MBps link to your fileserver, a number of 100-megabit clients can achieve wire-rate file access simultaneously.
Even without gigabit switching, NIC bonding should be a staple. At its simplest, bonding two NICs gives you redundancy, but add transmit load-balancing, and you can effectively double the outbound bandwidth. Using switch-assisted teaming will provide the same effect on inbound traffic. Almost every major server vendor offers NIC teaming drivers -- and there are third-party utilities, too. It’s a big, cheap bandwidth boost.