Eric Siebert published an very nice article about Memory usage in vCenter Server 4.0:

If you are using vCenter Server and plan on upgrading to vSphere you should be aware that vCenter Server 4.0 uses much more memory than vCenter Server 2.5 does. According to the official documentation for both 2.5 and 4.0, the system requirements are very similar:

• 2.0 GHz or faster Intel or AMD x86 processor. Processor requirements can be more if your database runs on the same hardware.
• 2 GB RAM. You might need more RAM if your database runs on the same hardware.

But VMware has a note in the vSphere documentation that states:

vCenter Server includes a service called VMware vCenter Management Webservices. This service requires128MB to 1.5GB of additional memory. The vCenter Management Webservices process allocates the required memory at startup.

This Webservices component is a Windows service called VMware VirtualCenter Management Webservices. It’s a Tomcat Java application server that is a component of the vCenter Server. You will see this service listed in Task Manager as tomcat6.exe as vCenter Server 4.0 uses the newer Tomcat 6 version. In vCenter Server 2.5, this service was listed as tomcat5.exe, vCenter Server 2.5 uses the older Tomcat 5.5 version. The Webservices component is not used just for Web browsers to access the vCenter Server, many other services rely on this component. If you stop this service component, vCenter Server will still work but anything that relies on Webservices will no longer work (i.e. plug-ins, Converter).