Generally all you need is 100Mbit. Most hard drives read at about 35-40Mbytes/second. Gigabit networking is 1000Mbits/s, or 125Mbytes/s. You would be using less than half of the bandwidth unless using scsi drives or direct from ram.
I would run a test on your network at 100Mbits first. A tool like "DU Meter," or XP's network usage graph are very helpful in finding out how fast stuff is flying across your network. My average network speeds is about 8Mbytes/s from another computer's hard drive. (that's about 70% network usage @ 100Mbits switched.) For me, adding 10x bandwidth to that is a waste of money.