Have a site that has already used 1.5 gigs this month.
Daily hits about 15000 page requests according to the report magic program.

Anyone know of a way to predict future bandwidtrh usage?

Most people use the WAG method :D.

Really, the best way to anticipate growth is to base it on past trends. Monitor your site growth over at least a 6 month period and see how much you have grown from month-to-month. Discount any anomolies (a sudden spike in traffic because of a special mailing, etc), and keep in mind overall traffic trends (like the fact that Internet traffic, in general, falls off during the summer).

Using this information you should be able to predict your grwoth fairly accurately.

Yeah, in the past, it has done betwee 7-8 gigs. Now with some recent publicity, been keeping an eye on it.