Check out one of the latest white papers from VMware. This 12 page document lists the metrics available for monitoring from VirtualCenter. It includes guidelines for setting the Statistics Collection Level, and indicates the types of metrics available for each level. It reads:
VMware Infrastructure 3 includes a system for gathering extensive metrics on performance, resource utilization, and basic statistics of all parts of the environment, including virtual machines, ESX Server hosts, resources pools, and host clusters. These metrics can be viewed using the VI Client, and are also available to external programs via the VMware Infrastructure SDK. All measurements may be obtained in real-time, or from a historic record.
The real-time metrics are obtained by connecting directly to the source of the data and transmitting the desired measurements as they are collected. The metrics are collected every 20 seconds for ESX Server 3 hosts, and every 60 seconds for ESX Server 2 hosts. They can be viewed through a VI Client session with VirtualCenter or directly with an ESX Server
host; in the latter case, only metrics pertaining to that host or the virtual machines on it are available.
The historical metrics are drawn from the VirtualCenter database, and can only be viewed with a VI Client connected to the VirtualCenter server, or a program that uses the VMware Infrastructure SDK. VirtualCenter aggregates the value of each metric over successively larger time intervals, using smaller intervals to generate values which are then rolled up into larger intervals, using either an average, summation, latest, maximum or minimum value over the interval.
The historical data available depends upon the following factors:
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The Statistics Collection Level
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The Statistics Collection Interval
Read the entire white paper, here.