How can the maximum allowed bandwidth limits be calculated? Lets say there is a standard Intel Celeron 2 GHZ processor on the server- how can this be used to calculate the amount of bandwidth you could promise each individual customer?
In just about all cases, the processor used does not accurately reflect the bandwidth used. For example, with fairly static pages - especially download files/images, that Celeron you mentioned could easily push several hundred Mbps. However, if your pages are dynamic and use a lot of server resources to be generated, then (depending of course on memory usage, etc as well) the Celeron you mentioned would likely keel over well before that. Therefore, bandwidth usage is much more dependent on the type of websites you're hosting, the amount of traffic they get, what kind of files are being accessed, and the resources those hits take to be served. If you're looking to host other sites, you should estimate the number of sites you're planning to allow per server, and then budget accordingly for both server hardware and bandwidth. Similarly, if you already have a budget in mind for your server, then you'll need to figure out how many sites you can host with the resources you have available. Unfortunately, this does require a bit of guesswork and a lot of past experience, so if you're starting anew, there's not a lot of previous customer data you can draw upon. Some educated planning, and the ability to adapt as you learn more about your individual customers' usage, is key. As you grow, the usage of each individual customer will become less important, as you get to average usage out among all of your customers.
Also, the allocated network port speed determines the theoretical limit of transfer you can expect in ideal cases.
Like bilal@revolutionhosting said, it's pretty much a guessing game unless you have some past performance numbers to go by. But to prevent nasty bandwidth overage fees, I would implement some sort of per account throttling until you have a good idea on what your actual usage will be. More than likely, 20% of your users will use 80% of your bandwidth.