I'd suggest taking your desires, budget, and requirements, and make a post in the hardware Section of the XBMC forums.
A simple NAS enclosure is your best bet. I was looking at them a while ago and the Synology enclosures were well priced while stil having good specs. WIreless N is enough to stream 720p from multiple sources, but can often struggle with 1080p. You can slo set up a cheap and/or old motherboard, cpu, ram, and hdd as an adhoc NAS and shove it into a closet
Check out the
XBMC Forums for lots of good info on this stuff. Plus XBMC is a wonderful program for watching and listening to various media.
Me, I crammed my old quad core CPU into a smaller, component size-compliant case with a few harddrives, adn store about 3000 TV episodes and 400 movies on it. Hardwired with gigabit NICs and cat6 cables. XBMC is very godo at sorting media and grabbing episode info/art from IMDB adn TVDB.com, and it has ALL codecs built-in. I suggest the "Transperensy!" skin.
If you wish to to an actual PC for the media and don't want a Windows OS, Unraid or Ubuntu are your best bet.
You can also find tiny HT-centered PCs that can be foudn cheap, adn work very well, like these:
http://www.newegg.ca/Barebone-Mini-Computers/Category/ID-3
PS - for ripping your media, Handbrake is free, has many options, and is pretty easy to use. Or use Imgburn is you wish to keep the menus intact.
PPS - if you do everything from scratch, I'd suggest a ANS for storage, and either pre-built HTPCs or those media players like Boxee, EDTV, Popcorn Hour, etc. for each television.
-- Of all the soles I've ever tasted, his had the most ....cumin. --