all I can give you is my experience. At home I am using the Linksys Wireless-G network router, this has a built in ADSL BB modem. My laptop I connect to the router via a wireless 802.11g PCMCIA adaptor, but there are 4 network ports on the router, so you can physically connect computers using traditional Cat5.
The router acts as the DHCP server for the computers, but this selectable in the setup, you can choose to have the computers set up so they comunicate directly with each other, with fixed IP addresses. I assume this would be the case if you wanted to have a home network for gaming and such like. Although I think you would need to tinker with DNS to make this work.
If you are talking about chaining your computers together I still think you would need a hub as your would only be able to get 2 computer yakking to each other using host and peer connections. With regards to the sharing of the BB connection the internet sharing wizard should sort that out for you.
On the next point I am more than willing to be corrected, but if you chain the computers and access the internet through one machine then you would only need the firewall on the main machine, but anti-virus on all computers. This is because the internet facing side of your network would be the main machine, which your firewall would protect, but the anti-virus would be need on all machines because if a chained machine downloaded a file it would be passed to that machine.
Very similar to an article in PC Pro some months ago about setting up your own firewall PC.