100Mbps unmetered is about 30 TB/month
1Gbps unmetered is about 300 TB/month
Those are maximum values...
Achieving those is totally different story and depends on too may factors...
If you do not have bigger files (downloads, video/audio streaming etc) 100Mbps is probably fine, but if you do then 1Gbps line is prefered since you can peak out of 100Mbps and users with faster connections can get those big files quicker without "blocking the line" to other visitors.
If you want to push much bandwidth - you also need powerful hardware and well optimized software.
In case you need any help or advices feel free to contact me at
celebempire{at}gmail.com
Here's the list of the things I can help you with
http://www.awmzone.com/services