Vista Ultimate, because of a small section of the Retail Software License on page 13:

Code:
MICROSOFT WINDOWS VISTA ULTIMATE SERVICE PACK 1
ADDITIONAL LICENSE TERMS. The following additional license terms apply to Microsoft Windows
Vista Ultimate.

2. NETWORK USE. Instead of installing the software on the licensed device, you may install one copy
on a storage device, such as a network server. You may use that copy only to run the software on
your licensed device over an internal network.
It probably would have been cheaper to buy the extra hard drives instead of an Ultimate license, but I really don't like noise.