Bandwidth is sold several different ways in a colocation environment. Three of the most common ways are 95%, TB/GB and unmetered.
Most colo providers have gone to 95% since that's how they are billed. It makes pricing more competitive since you're only paying for what you're using. You can read more about 95% billing here:
http://www.seanadams.com/95/ . Some providers, Cogent for example, have even started doing 90% billing. It's the same as 95%, but the top 10%, instead of 5%, is thrown out. If you have jerky traffic, stay away from 95%.
Some providers still offer GB/TB plans. As Hollanda mentioned, the general conversion rule is ~320 GB = 1 Mbps. The problem with this type of billing, for providers, is you can transfer much less than 320 GB per month and have a 95% that’s much higher than 1 Mbps, yet they have to pay for the 95%. They end up taking it in the pants by having to pay out more money than they’re charging you and that’s why many providers have tried to step away from GB/TB billing. If you have jerky traffic, this is definitely the way to go. You'll get much more for your money.
Another common option is "unmetered". With an unmetered plan, the port you're plugged into is usually capped at a certain speed. There's no 95%, no transfer limits, nothing like that. You can transfer as much as you want, whenever you want, but you'll only be able to send data at whatever your port has been capped/throttled to. This is not how 95% and GB/TB plans operate. The other types I already mentioned will give you burst speeds of 10 or 100 Mbps, depending on provider. Unmetered 5 Mbps is usually going to be much cheaper than 5 Mbps on 95%, but you aren’t going to get the capability to burst to 10 or 100 Mbps. That can definitely be a problem when you host a significant amount of slots.
As for your question about how much bandwidth you'll need, Hollanda hit it on the head. It depends on how often the servers will be full.