From the Dreamhost KBase:
[color=#CC0000]Overage is calculated and charged per month.
DreamHost will not automatically shut down sites that exceed their limit. In the unlikely case that your site exceeds its allocation, you will be notified and offered the opportunity to pay for more bandwidth. You will always be contacted before any action is taken.
Of course if you are consistently using more bandwidth than is allowed by your plan, consider upgrading![/color]
Hope that helps