Might just need your help mate ;)

Im looking for a way to cap monthly bandwidth limits on certain directories on multple domain acounts. For example -

I have 5 acounts, i set up a "download" folder in each of the acounts public_html directories. I want to limit them to 20 gig a month each, but leave the rest of them will full bandwidth.

I want to be able to do this so i can keep the websites online, and also not go over my servers bandwidth limit, so the heavy downloads (audio mainly) will stop untill the next month.

The sever is going to be Fredora core 2, running cpanel, and apache. The only way i can think is a script in apache somewhere, but i havent got a clue.

Can you help?