The main unit is an HP EX490 storage server with 2 x 1.5TB WD15EADS and 2 x WD20EARS drives, and the CPU upgraded to an Intel E6300 2.8GHz dual core. This machine runs an iTunes client to serve up around 1TB of media to an AppleTV, hosts two Time Machine backups, one for for each of my Macs, hosts a backup of my photo library plus backups of other files. The external drive dock is connected via eSATA to the HP server and I use it to back up the storage server to bare drives that are stored elsewhere. The big external is a LaCie 2Big triple configured as 1TB of RAID 1, and it hosts a bootable clone of my Mac Pro's primary drive. The small external is a Seagate 320GB 7200rpm drive in a bus-powered FW800 Rosewill enclosure that hosts a bootable clone of my MBP's drive.
Update:
I replaced the EX490 with a Synology DS411 last Fall. I am still using the eSATA drive dock for server backups in conjunction with several WD green drives: a 2.5TB and 2 1.5TB drives. I stuck the 320GB 2.5" drive into a USB 3.0 enclosure from OWC for use with my new MBA.