Thank you for the answer about multisites, I need to read up on those. I had looked into parent/child sites, but that seems to not be the same thing, mostly relating to themes.
What I'm mainly curious about, though, is:
a) If I install a multisite network on our Dreamhost domain, will upgrades to that installation appear on every site?
b) Does each site appear unique and separate, with different navigation, than the others?
c) Can I make changes to the theme for one site without affecting the other sites?
Thanks for all the help you are giving me.