For the last few years I've been using a Thecus N5200 NAS storage system. I've been very happy with it, but I've actually underutilized it. It has five drive bays, and I've only ever had three 500GB drives in it running in a RAID 5 configuration. That's a total of just under 1TB of usable space.
Well, the price of hard drive storage has gone way down, and I finally decided to go the distance. I purchased five 1TB Western Digital Caviar Green drives from Amazon for a total of $424.95. Not bad!
Haven't installed them yet, but it will be nice to have 4TB of usable space!