How can site collections (top-level WSS sites and their sub sites)
be moved from one content database to another?

Details about our installation:
We are using both WSS and SPS 2003.
When we originally build our infrastructure, we started with a
single content database. Eventually, we added 2 more content
databases.
We would like to cap our content database sizes to 35 GB.
After we added the new content databases, all new sites have been
built on the newer content databases. But, the original content
database continues to grow in size as the earlier sites continue to
grow.

In order to manage these content databases, we'd like to move sites
from the 'full' database to the newer content databases.

Can we move site collections and retain their content, security
settings and URLs?

Along these same lines, is there a method for changing site
collection URLs?