I really don't think you'll have a problem with going over. I used to have an air card with Verizon and with moderate to heavy use I never went over. In fact, the most I ever used was a little over 3gb when my cable internet service went down and that's all I had for internet at home. Since it was stolen (aircard) I've been using my BB as a tethered modem and have not gone over 2gb in a month. I use it off and on all day out in the field. Mainly work stuff, emails but the occasional goof offs like this site
as well as other forums I vist that all have these animated ads etc.
So, like I said, I doubt you'd go over on the handheld itself. In fact, my buddy has a Storm and he watches youtube clips all day long, web surfs etc and he's never gone over or been reprimanded by VZW.
EDIT: Forgot to add that I use Podtrapper on my BB and I download two podcasts (talk radio) everyday. Each one has three audio files of about 12-15mb for each hour or file. I do this everyday M-F and have for about six months without going over being reprimanded by VZW. Also use/used iheart radio, slacker etc without worrying about going over.
So, to make a long story short, don't worry about it...;)