It depends on 2 factors.
- How many clients are using the zone(s) your server will be in.
- How many other servers are in the same zone(s) and their bandwidth setting.
The bandwidth setting on the web page is actually a weight. While the values relative to themselves are accurate (i.e. You will see double the bandwidth usage going from 50Mb to 100Mb.), In terms of how much actual bandwidth usage will be used is an unknown because of the two factors I mentioned above.
Choosing 384k or 512k keeps your server out of the “global” pool, and only in the continent / country. Depending on your bandwidth and location it’s always best to start at a low value and you can bump it up incrementally to suit how much usage you are looking to provide.