I have both a wired and wireless networks within my house, each of the main rooms (living room, convertory, office, bedrooms) has a single ethernet cable run to it. Where the room requirements dictate it I have a simple ethernet switch to give me the extra ports I need. In my office I have a 20 port switch hooked up to my ADSL router/firewall - this also provides my IP settings.
I also have a 802.11g wireless network, but this tends to only get used for my laptops, I found that it dropped frames during movie playback, despite getting at least a medium strength signal anywhere in the house/garden. *this could be due to the large amount of wireless devices in my house or my choice of wireless gear - all netgear*
I have a Linux File Server (Athlon 2400+, 256 MB RAM, 750 GB Disk) that acts as a file server for my Desktops/Laptops and as my streaming server to my Xboxes.
I use the XBMS protocol (requires the ccxstreaming server) to push video/pictures/music to each of the Xboxes, it acts much like a simple, unsecured, fileshare (you can put a password on the share but its sent plain text so its not really secure). Its biggest benefit over using a windows style share is that it uses much less bandwidth - which is important when you are streaming 3 * DVDs at 6 mbit per second (as is the transfer rate of your IDE bus).
I only run on Linux as its free - I won't pay for or pirate Windows (I'd get the sack for sure if I did that) - you can run this on a windows PC, its actually easier as there is a nice little GUI program to configure everything for you:
http://dott.lir.dk/ccxgui/
The Xboxes run Xbox Media Center (make sure you are running the may release, it fixes so many bugs), and have been configured (in mediacenter.xml - I think) to point to the XBMS share. On the Xboxes upstairs Xbox Media Center is the default dash - basicly to stop the kids messing, downstairs I use avalaunch, then manually load media center when I want to.
As previously mentioned, Media Center can point to a Windows file share, but you must set the Windows share to support plain text passwords (this is not the default in XP) or allow anoymous access to your share.
Once you have your share setup and accessible to Media Center, you then need to browse to it, and then run the scan task against it. This should find all of your media for each type - video, pictures, music. You can scan multiple shares and Media Center will list them all as a single list - this is useful if you need to load balance your movies across multiple PCs/shares.
NOTE: This could take some time, about 20 minutes for all my MP3s (about 120 GBs worth)
And thats about it. If you have any spesfic questions I'll try to answer them, I'm also going to try (can't promise) to do a proper FAQ complete with screenshots for this if there is enough demand.