Hmmm, 400K hits a day might be an achievement but I have certainly seen HUN listings pull down in excess of 250 Gbs/day .
Secondly the fact that your host can't grow with you would also be a concern to me. You have been developing your site for awhile would be my guess and the traffic has grown for your efforts but can you imagine the effect on all of your hard work if you were down for a day or more because you had exceeded your BW limit?
A quality host should be able to (and in fact be happy about) scale as you grow. If nothing else make sure you have a migration plan in place if you see yourself coming even close to their limit.
Good luck an please do keep
http://www.Caro.Net in mind if you ever happen to decide to make a move.