Newegg has a 4 drive unit that does RAID 0/1/5 for around $500-$600. You just need to add the drives, and they have 500GB seagates (with 5 year warranty) for $140 now.If you need really crazy storage you can build a linux based NAS from one of those MATX cases and how ever many hard drives you can get it to fit.
I have a 3Tb external NAS for media storage (Full DVD collection, music, TV shows, etc) that I love. Unfortunately it was a pain in the ass to build and set up, so it's not for the faint of heart. Much cheaper to build it yourself than to buy it. Most NAS's sell for $1200 or so for a TB of storage, but for that same $1200 I built my 3Tb storage unit.
That is what I got, the seagateNewegg has a 4 drive unit that does RAID 0/1/5 for around $500-$600. You just need to add the drives, and they have 500GB seagates (with 5 year warranty) for $140 now.