Umm yeah all good ideas, expensive and somewhat troublesome, but still good USB drive, expensive, and innefficient. NAS, expensive and not as easy to upgrade it or multi-task it.
My suggestion, build a pc to be a file server, if it is just going to be a file server you can get away with a 300mhz processor, you can run raid for redundancy, and can upgrade the space with more and more controllers and drives, personally though you can make it do other stuff, mine is a dual 1.3ghz with 512mb of ram, mail, dns, dhcp, web, ldap, and obviously file server, even with all this load rarely goes above 10%. I currently have one 2port ide raid controller in it that I got for $30, running 4 120gb hard drives(I got for between $150 and $50), not in raid yet, as I need to empty them first. Also going to get a 8-port 64bit(as this board supports 64bit pci, for faster access) sata raid controller for $150 and get 8 300gb drives in raid 5 for ultimate storage.
Mostly this is used for linux isos, music, sharing dvd's and backing up laptops and desktops in our network. We also have over 50gb of photos... With all this centralised I can burn the whole lot to a stack of dvdrw's every month and leave them at work just in case the unthinkable happens.
Oh and about your question of bandwidth required, I have succesfully run a dvd from the server across the lan and across my wireless lan to a laptop with no issues. most mp3's are at max 256kbs thats one-quater of a Mbps, video maxs out at around 2500kbps, or 2.5(roughly)Mbps, again a 100mb link easily handles it.