What bogs down a database driven web site are 2 things:
1. # of connections to the database-
This makes sense, the more people on the board, the more times it is connecting, the more time it takes up. If you are logged in or logged out it really doesn't matter.
2. amount of data being called from the database
So, to answer your question SandyD, if you were to reduce the amount of days worth of posts you look at, it COULD theoretically reduce the bandwidth. That is less data that needs to be pulled every time you look at the front page. If enough people that visit the site repeatedly throughout the day did this, it could make a difference.