A) Are you going to be doing HD or SD in FCP? Cause if you're doing HD, FW400 is going to be too slow. For best results, you will need to setup a RAID box. It can be connected over ethernet (probably gigabit though, not regular ethernet). As well, you might want to look at Final Cut Server which is software for managing assets from a common media server by numerous FCP clients.
B) A single computer will not be able to stream multiple HD streams to 30 screens, it's just too much content. Depending on the HD content, you can get a way with a dozen or so (don't quote me as I don't remember exactly how many you can stream with current Mac Pros). As far as output, you can get an AJA or Blackmagic PCI card that has SDI, HD-SDI, component, composite, etc. outputs that you can attach to your screens, repeaters, etc. You'd need software to send a video signal out though. Both AJA and Blackmagic cards usually come with a basic output app (AJA TV, etc.) that can send a video out.