VMware VirtualCenter uses standard the Micrososft SQL Express 2005 database. The statement of VMware is:
Microsoft SQL Server 2005 Express is intended to be used for small deployments of up to 5 hosts and 50 virtual machines
Microsoft SQL 2005 Expresss supports 1 CPU, 1 GB of RAM and a maximum database size of 4 GB.
There is a nice SQL Server 2005 Features Comparison page table available between Microsoft SQL Express, Workgroup, Standard and Enterprise link. Thanks ablej for pointing.