Suppose I buy the Bronze plan, which allows me a 20.000 MB monthly data transfer. The same month, my website becomes a huge hit and data transfer exceeds that amount, say 50 GB. What happens then?

a. my users will be denied the download of data onde I reach the 20GB quota?
b. will I be charged by the GB?
c. will I be charged the Silver plan, which includes 150GB of data?
d. none of the above (what then?)

This should really be in the knowledge base.

as with any host if you exceed the bandwidth or space of your account, your account will be suspended until the resources are corrected.

In the case of disk space this is total disk space so you will need to delete items to continue or upgrade to a new package. BigBlueHost will not upgrade you or charge you additional amounts without authorization from the client.

To upgrade you can purchase an add on feature

http://www.bigbluehost.com/hosting-addon.html
or you can make payment here to a new package
http://www.bigbluehost.com/order-page.html

In the case of bandwidth your account limit resets each month and will have to either wait until the end of the month or again upgrade your package. In both cases you are emailed numerous warnings telling you your space and bandwidth will exceed. If you need to upgrade we can get that done in about 20 minutes. The only way your site will go down is if you ignore the warnings and do nothing.