Are you sure you need a gigabit adapter? If anything, it might be that you need more RAM and or CPU power to host all the minecraft sessions.
What is your current ISP's upload and download speeds? A gigabit adapter probably won't do much for you if you have less than a 100Megabit connection.
ive got 16gbs 1600mhz ram and a 3.6ghz processor also a 60gb ssd to run the server off