On the link provided, don't miss to click the "Compare Kits" link. VMware offers a couple of kits which already include vCenter Server. The two Essential Kits are low priced all-in-one kits which cannot be expanded, and the Acceleration Kits are basically starter kits which can be expanded with additional CPU licenses as needed. The http://www.vmware.com/files/pdf/vsphere_pricing.pdf provides information about the different editions and kits.
André