Witness my external hard drive, and quake in your boots.
3x300GB SATA drives in RAID5 attached to a RocketRaid 2220 pci / pci-x card from ebay. Plus a cheap gigabit ethernet card.
Soon to have a 4th 300Gb SATA drive inserted.
1TB (roughly) SATA RAID 5, webserver, media server, backup storage, video editing storage, running OS X for about £400. Plus room on the raid card for a further 4x SATA disks to take it up to 2 TB or whatever.
It feeds my main workstation, a PBG4 1.5ghz, plus a small deaf arts charity of about 4 staff, equipped with ibooks.
If I had to do it again, I'd pick a cheap Powermac G4 - installed gigabit, stronger power supply, better energy management and wake-on-lan would help a lot.