Steve, it really depends on what type of traffic you are serving. For instance, if you are looking at TGP galleries, your picture quality will be lower and on average visitors will only click on about 1/3 of your thumbs so you can effectively serve 50,000 page hits a day and burn less than 200G a month. However, if you are running a paysite and offering video downloads, 100 members can easily burn the same 200G each month.

Like Lee said, tell us about what you want to do and maybe we can give you a better estimate.

cheers!
Luke
(ps - Michael, thanks for the kind words!)