Good question, are these domains/websites established? How many vistors do you see in a day? Doesn't your current host have stats to provide you these answers? Its really hard to say depending upon the nature of the site.
It depends on how many visitors you get a day and what is on your site. I wouldn't have a clue with the first post
How much bandwidth do you take a day from that site? We can guess the monthly amount needed from that information.