Just for a testing purpose I used Xen almost a year to consolidate my dedicated server, initially dual core and now quad core. First I used Xen VM migration feature which was allowed me to have a separate solution on a server to use DEV. After switching to Core2Duo, I took an advantage of virtualization for consolidation and disconnected the the four large machines that are now just more than a small NAS.
Recently, I decided to upgrade to a Core2Quad before the technology changed. Even though the principle defined for motherboard / cpu to 45nm demands high end servers.