You absolutely MUST use gigabit cards for network rendering.
I have a dedicated fileserver connected to my SAN plus two additional Vegas rendering servers. These three are connected together using teamed NICs giving me 4Gb/s. The editing suite is connected to this network using standard 1Gb/s network card (Broadcom NetXtreme).
The next upgrade I will make will be to a network card that off-loads the TCP work from the CPU. Similar to use a high performance graphics card this will remove substantial workload from the CPU leaving more cycles for the application.
A high performance network card also needs high performance switches; we use HP ProCurve here.
Mike