There are many choices in Servers depending on your needs.
Systems with the S5500WB are ultra energy efficient, but you give up expanction and PCI-e slots.
Pedital or rack?
This link will take you to the Intel Sever page where you can get a good ideal of what is advaiable from Intel.
All will run Xen and I am very partial to dual Xeon Westmere 6 core hyperthread processors.