Most of my machines have 1TB drives and 2TB external backup drives.
I was thinking about moving to multiple Virtual Machines running on the desktop PC with a 4TB drive to hold the VMs and another for backups.
This means losing having the SSD as Windows 7 boot drive unless I get a bigger SSD for Windows 7 and the Hypervisor and keep Linux machines on a HDD.
Still, as of today I know what the server needs for VM backups - Seagate brings out 6TB HDD
Get a datasheet here (PDF).