To Collect Virtual Infrastructure information, you'll need to be on VMware 4.0 or better. (This applies to vSphere, vCetner, or ESX[i]).
In the Administration Guide (starting on Page 185), there are details on adding your virtual devices. If you add a vCenter or vSphere, then you'll need to assign collectors (after the initial polling) to your various agents for each ESX Host. There's more information later in the Admin Guide about how to balance agents and workload (see Chapter 5 specifically).